{"id":177,"date":"2021-02-11T19:38:06","date_gmt":"2021-02-11T19:38:06","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-hvcc-healthpsychology\/?post_type=chapter&p=177"},"modified":"2021-02-17T02:21:49","modified_gmt":"2021-02-17T02:21:49","slug":"sleep","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-hvcc-healthpsychology\/chapter\/sleep\/","title":{"raw":"Sleep","rendered":"Sleep"},"content":{"raw":"Sleep<\/b>\u00a0is a naturally recurring state of mind and body, characterized by altered\u00a0consciousness, relatively inhibited sensory activity, reduced\u00a0muscle\u00a0activity and inhibition of nearly all\u00a0voluntary muscles\u00a0during\u00a0rapid eye<\/strong> movement\u00a0(REM) sleep,[1]<\/a>\u00a0and reduced interactions with surroundings.[2]<\/a>\u00a0It is distinguished from\u00a0wakefulness\u00a0by a decreased ability to react to\u00a0stimuli, but more reactive than a\u00a0coma\u00a0or\u00a0disorders of consciousness, with sleep displaying very different and active brain patterns.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"603\"]\"\" Sleep is associated with a state of muscle relaxation and reduced perception of environmental stimuli.[\/caption]\r\n\r\nSleep occurs in\u00a0repeating periods<\/a>, in which the body alternates between two distinct modes:\u00a0REM<\/a>\u00a0sleep and\u00a0non-REM<\/a>\u00a0sleep. Although REM stands for \"rapid eye movement\", this mode of sleep has many other aspects, including virtual\u00a0paralysis<\/a>\u00a0of the body. A well-known feature of sleep is the\u00a0dream<\/a>, an experience typically recounted in\u00a0narrative<\/a>\u00a0form, which resembles waking life while in progress, but which usually can later be distinguished as fantasy. During sleep, most of the\u00a0body's systems<\/a>\u00a0are in an\u00a0anabolic<\/a>\u00a0state, helping to restore the\u00a0immune<\/a>,\u00a0nervous<\/a>,\u00a0skeletal<\/a>, and\u00a0muscular<\/a>\u00a0systems; these are vital processes that maintain mood, memory, and cognitive function, and play a large role in the function of the\u00a0endocrine<\/a>\u00a0and\u00a0immune systems<\/a>.[3]<\/a><\/sup>\u00a0The internal\u00a0circadian clock<\/a>\u00a0promotes sleep daily at night. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research.[4]<\/a><\/sup>\u00a0Sleep is a highly\u00a0conserved<\/a>\u00a0behavior across animal evolution.[5]<\/a><\/sup>\r\n\r\nHumans may suffer from various\u00a0sleep disorders<\/a>, including\u00a0dyssomnias<\/a>\u00a0such as\u00a0insomnia<\/a>,\u00a0hypersomnia<\/a>,\u00a0narcolepsy<\/a>, and\u00a0sleep apnea<\/a>;\u00a0parasomnias<\/a>\u00a0such as\u00a0sleepwalking<\/a>\u00a0and\u00a0rapid eye movement sleep behavior disorder<\/a>;\u00a0bruxism<\/a>; and\u00a0circadian rhythm sleep disorders<\/a>. The use of\u00a0artificial light<\/a>\u00a0has substantially altered humanity's sleep patterns.[6]<\/a><\/sup>\r\n

Physiology<\/span><\/h2>\r\n
Main article:\u00a0Neuroscience of sleep<\/a><\/div>\r\n
\r\n
\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"200\"]\"\"<\/a> An artist's creative illustration depicting REM sleep[\/caption]\r\n\r\n
\r\n
<\/div>\r\n
The most pronounced physiological changes in sleep occur in the brain.<\/span>[7]<\/a><\/sup>\u00a0The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of\u00a0<\/span>adenosine triphosphate<\/a>\u00a0(ATP), the molecule used for short-term storage and transport of energy.<\/span>[8]<\/a><\/sup>\u00a0In quiet waking, the brain is responsible for 20% of the body's energy use, thus this reduction has a noticeable effect on overall energy consumption.<\/span>[9]<\/a><\/sup><\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nSleep increases the\u00a0sensory threshold<\/a>. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events.[9]<\/a><\/sup>[7]<\/a><\/sup>\r\n\r\nDuring\u00a0slow-wave sleep<\/a>, humans secrete bursts of\u00a0growth hormone<\/a>. All sleep, even during the day, is associated with secretion of\u00a0prolactin<\/a>.[10]<\/a><\/sup>\r\n\r\nKey physiological methods for monitoring and measuring changes during sleep include\u00a0electroencephalography<\/a>\u00a0(EEG) of\u00a0brain waves<\/a>,\u00a0electrooculography<\/a>\u00a0(EOG) of eye movements, and\u00a0electromyography<\/a>\u00a0(EMG) of\u00a0skeletal muscle<\/a>\u00a0activity. Simultaneous collection of these measurements is called\u00a0polysomnography<\/a>, and can be performed in a specialized\u00a0sleep laboratory<\/a>.[11]<\/a><\/sup>[12]<\/a><\/sup>\u00a0Sleep researchers also use simplified\u00a0electrocardiography<\/a>\u00a0(EKG) for cardiac activity and\u00a0actigraphy<\/a>\u00a0for motor movements.[12]<\/a><\/sup>\r\n

Non-REM and REM sleep<\/span><\/h3>\r\nSleep is divided into two broad types:\u00a0non-rapid eye movement<\/a>\u00a0(non-REM or NREM) sleep and\u00a0rapid eye movement<\/a>\u00a0(REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is called\u00a0slow-wave sleep<\/a>\u00a0or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy.[7]<\/a><\/sup>\u00a0REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for\u00a0dreams<\/a>\u00a0(or\u00a0nightmares<\/a>), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone,[2]<\/a><\/sup>\u00a0and suspension of\u00a0homeostasis<\/a>.[13]<\/a><\/sup>\r\n\r\nThe\u00a0sleep cycle<\/a>\u00a0of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4\u20136 times in a good night's sleep.[12]<\/a><\/sup>[14]<\/a><\/sup>\u00a0The\u00a0American Academy of Sleep Medicine<\/a>\u00a0(AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or\u00a0slow-wave sleep<\/a>.[15]<\/a><\/sup>\u00a0The whole period normally proceeds in the order: N1 \u2192 N2 \u2192 N3 \u2192 N2 \u2192 REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep.[2]<\/a><\/sup>\u00a0There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.[12]<\/a><\/sup>\r\n

Awakening<\/span><\/h3>\r\n
<\/div>\r\n
\r\n
\r\n\r\n[caption id=\"\" align=\"alignleft\" width=\"220\"]\"\"<\/a> \"The Awakening\", an illustration to writing by\u00a0Leo Tolstoy[\/caption]\r\n\r\n
\r\n
Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal\u00a0<\/span>circadian indicators<\/a>, along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle.<\/span>[16]<\/a><\/sup>\u00a0Awakening involves heightened electrical activation in the brain, beginning with the\u00a0<\/span>thalamus<\/a>\u00a0and spreading throughout the\u00a0<\/span>cortex<\/a>.<\/span>[16]<\/a><\/sup><\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nDuring a night's sleep, a small amount of time is usually spent in a waking state. As measured by electroencephalography, young females are awake for 0\u20131% of the larger sleeping period; young males are awake for 0\u20132%. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13\u201314% in the fifth. Most of this awake time occurred shortly after REM sleep.[16]<\/a><\/sup>\r\n\r\nToday, many humans wake up with an\u00a0alarm clock<\/a>;[17]<\/a><\/sup>\u00a0however, people can also reliably wake themselves up at a specific time with no need for an alarm.[16]<\/a><\/sup>\u00a0Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization.[18]<\/a><\/sup>[17]<\/a><\/sup>\u00a0Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle.[19]<\/a><\/sup>[20]<\/a><\/sup>\u00a0Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying\u00a0sleep inertia<\/a>.[21]<\/a><\/sup>\r\n

Timing<\/span><\/h2>\r\nSleep timing is controlled by the\u00a0circadian clock<\/a>\u00a0(Process C), sleep-wake\u00a0homeostasis<\/a>\u00a0(Process S), and to some extent by the individual will.\r\n

Circadian clock<\/span><\/h3>\r\n
Main article:\u00a0Circadian rhythm<\/a><\/div>\r\n
Further information:\u00a0Circadian rhythm sleep disorder<\/a><\/div>\r\n
\r\n
\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"650\"]\"\"<\/a> The human \"biological clock\"[\/caption]\r\n\r\n
\r\n
<\/div>\r\n \r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nSleep timing depends greatly on\u00a0hormonal<\/a>\u00a0signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day\u2013night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in\u00a0diurnal<\/a>\u00a0animals) and augments it at night.[22]<\/a><\/sup>[18]<\/a><\/sup>\u00a0The\u00a0suprachiasmatic nucleus<\/a>\u00a0(SCN), a brain area directly above the\u00a0optic chiasm<\/a>, is presently considered the most important nexus for this process; however, secondary clock systems have been found throughout the body.\r\n\r\nAn organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be\u00a0entrained<\/a><\/i>; an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period that slightly exceeds 24 hours. Scientists refer to such conditions as\u00a0free-running<\/a>\u00a0of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day.[17]<\/a><\/sup>[23]<\/a><\/sup>[24]<\/a><\/sup>\r\n\r\nThe circadian clock exerts constant influence on the body, affecting\u00a0sinusoidal<\/a>\u00a0oscillation of\u00a0body temperature<\/a>\u00a0between roughly 36.2\u00a0\u00b0C and 37.2\u00a0\u00b0C.[24]<\/a><\/sup>[25]<\/a><\/sup>\u00a0The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night.[26]<\/a><\/sup>\u00a0The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to the\u00a0pineal gland<\/a>, which releases the hormone\u00a0melatonin<\/a>\u00a0at night.[26]<\/a><\/sup>\u00a0Cortisol<\/a>\u00a0levels typically rise throughout the night,\u00a0peak in the awakening hours<\/a>, and diminish during the day.[10]<\/a><\/sup>[27]<\/a><\/sup>\u00a0Circadian\u00a0prolactin<\/a>\u00a0secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone.[10]<\/a><\/sup>\r\n\r\nThe circadian rhythm influences the ideal timing of a restorative sleep episode.[17]<\/a><\/sup>[28]<\/a><\/sup>\u00a0Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereas\u00a0slow-wave sleep<\/a>\u00a0can occur more independently of circadian time.[24]<\/a><\/sup>\r\n\r\nThe internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, at\u00a0the right moment<\/a>\u00a0in the circadian cycle, can significantly 'reset' the internal clock.[25]<\/a><\/sup>\u00a0Blue light, in particular, exerts the strongest effect,[18]<\/a><\/sup>\u00a0leading to concerns that\u00a0electronic media use<\/a>\u00a0before bed may interfere with sleep.[19]<\/a><\/sup>\r\n\r\nModern humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially\u00a0night shifts<\/a>), long-distance travel, and the influence of universal indoor lighting.[24]<\/a><\/sup>\u00a0Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely, they can have difficulty waking up in the trough of the cycle.[16]<\/a><\/sup>\u00a0A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise.[24]<\/a><\/sup>\r\n

Process S<\/span><\/h3>\r\n
Main article:\u00a0Sleep debt<\/a><\/div>\r\nGenerally speaking, the longer an organism is awake, the more it feels a need to sleep (\"sleep debt\"). This driver of sleep is referred to as\u00a0Process S<\/b>. The balance between sleeping and waking is regulated by a process called\u00a0homeostasis<\/a>. Induced or perceived lack of sleep is called\u00a0sleep deprivation<\/a>.\r\n\r\nProcess S is driven by the depletion of\u00a0glycogen<\/a>\u00a0and accumulation of\u00a0adenosine<\/a>\u00a0in the forebrain that disinhibits the\u00a0ventrolateral preoptic nucleus<\/a>, allowing for inhibition of the\u00a0ascending reticular activating system<\/a>.[29]<\/a><\/sup>\r\n\r\nSleep deprivation tends to cause slower brain waves in the\u00a0frontal cortex<\/a>, shortened attention span, higher anxiety, impaired memory, and a grouchy mood. Conversely, a well-rested organism tends to have improved memory and mood.[30]<\/a><\/sup>\u00a0Neurophysiological and functional\u00a0imaging studies<\/a>\u00a0have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.[31]<\/a><\/sup>\r\n\r\nThere is disagreement on how much sleep debt is possible to accumulate, and whether sleep debt is accumulated against an individual's average sleep or some other benchmark. It is also unclear whether the prevalence of sleep debt among adults has changed appreciably in the\u00a0industrialized world<\/a>\u00a0in recent decades. Sleep debt does show some evidence of being cumulative. Subjectively, however, humans seem to reach maximum sleepiness after 30 hours of waking up.[24]<\/a><\/sup>\u00a0It is likely that in\u00a0Western societies<\/a>, children are sleeping less than they previously have.[32]<\/a><\/sup>\r\n\r\nOne neurochemical indicator of sleep debt is\u00a0adenosine<\/a>, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine levels increase in the cortex and basal forebrain during prolonged wakefulness, and decrease during the sleep-recovery period, potentially acting as a homeostatic regulator of sleep.[33]<\/a><\/sup>[34]<\/a><\/sup>\u00a0Coffee<\/a>\u00a0and\u00a0caffeine<\/a>\u00a0temporarily block the effect of adenosine, prolong sleep latency, and reduce total sleep time and quality.[35]<\/a><\/sup>\r\n

Social timing<\/span><\/h3>\r\nHumans are also influenced by aspects of\u00a0social time<\/i>, such as the hours when other people are awake, the hours when work is required, the time on the clock, etc.\u00a0Time zones<\/a>, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. The approximate nature of the time zone can be shown with China, a country which used to span five time zones and now\u00a0officially uses only one<\/a>\u00a0(UTC+8).[17]<\/a><\/sup>\r\n

Distribution<\/span><\/h3>\r\nIn\u00a0polyphasic sleep<\/a>, an organism sleeps several times in a 24-hour cycle, whereas in monophasic sleep this occurs all at once. Under experimental conditions, humans tend to alternate more frequently between sleep and wakefulness (i.e., exhibit more polyphasic sleep) if they have nothing better to do.[24]<\/a><\/sup>\u00a0Given a 14-hour period of darkness in experimental conditions, humans tended towards bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Bimodal sleep in humans was more common before the\u00a0industrial revolution<\/a>.[27]<\/a><\/sup>\r\n\r\nDifferent characteristic sleep patterns, such as the familiarly so-called \"early bird<\/a>\" and \"night owl<\/a>\", are called\u00a0chronotypes<\/a><\/i>. Genetics and sex have some influence on chronotype, but so do habits. Chronotype is also liable to change over the course of a person's lifetime. Seven-year-olds are better disposed to wake up early in the morning than are fifteen-year-olds.[18]<\/a><\/sup>[17]<\/a><\/sup>\u00a0Chronotypes far outside the normal range are called circadian rhythm sleep disorders.[36]<\/a><\/sup>\r\n

Naps<\/span><\/h4>\r\n
<\/div>\r\n
\r\n
\r\n\r\n\"\"<\/a>\r\n
\r\n\r\nThe\u00a0<\/span>siesta<\/a>\u00a0habit has recently been associated with a 37% lower coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep.<\/span>[37]<\/a><\/sup>\u00a0Short naps at mid-day and mild evening exercise were found to be effective for improved sleep, cognitive tasks, and mental health in elderly people.<\/span>[38]<\/a><\/sup>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

Genetics<\/span><\/h3>\r\nMonozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Neurotransmitters, molecules whose production can be traced to specific genes, are one genetic influence on sleep that can be analyzed. The circadian clock has its own set of genes.[39]<\/a><\/sup>\u00a0Genes which may influence sleep include\u00a0ABCC9<\/a>,\u00a0DEC2<\/a>,\u00a0Dopamine receptor D2<\/a>[40]<\/a><\/sup>\u00a0and variants near\u00a0PAX 8<\/a>\u00a0and\u00a0VRK2<\/a>.[41]<\/a><\/sup>\r\n

Quality<\/span><\/h3>\r\nThe quality of sleep may be evaluated from an objective and a subjective point of view. Objective sleep quality refers to how difficult it is for a person to fall asleep and remain in a sleeping state, and how many times they wake up during a single night. Poor sleep quality disrupts the cycle of transition between the different stages of sleep.[42]<\/a><\/sup>\u00a0Subjective sleep quality in turn refers to a sense of being rested and regenerated after awaking from sleep. A study by A. Harvey et al. (2002) found that insomniacs were more demanding in their evaluations of sleep quality than individuals who had no sleep problems.[43]<\/a><\/sup>\r\n\r\nHomeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep.[44]<\/a><\/sup>[45]<\/a><\/sup>\u00a0Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.[46]<\/a><\/sup>\u00a0The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:[47]<\/a><\/sup>\u00a0maximum concentration of the hormone melatonin, and minimum core body temperature.\r\n

Ideal duration<\/span><\/h2>\r\n
\r\n
\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"372\"]\"\"<\/a> The main health effects of\u00a0sleep deprivation, indicating impairment of normal maintenance by sleep[\/caption]\r\n\r\n
\r\n
<\/div>\r\nHuman sleep-needs vary by age and amongst individuals; sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by\u00a0<\/span>actigraphy<\/a>,<\/span>[49]<\/a><\/sup>\u00a0and those affected with\u00a0<\/span>sleep state misperception<\/a>\u00a0may typically report having slept only four hours despite having slept a full eight hours.<\/span>[50]<\/a><\/sup>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nResearchers have found that sleeping 6\u20137 hours each night correlates with longevity and cardiac health in humans, though many underlying factors may be involved in the causality behind this relationship.[51]<\/a><\/sup>[52]<\/a><\/sup>[53]<\/a><\/sup>[54]<\/a><\/sup>[41]<\/a><\/sup>[55]<\/a><\/sup>[56]<\/a><\/sup>\r\n\r\nSleep difficulties are furthermore associated with psychiatric disorders such as\u00a0depression<\/a>,\u00a0alcoholism<\/a>, and\u00a0bipolar disorder<\/a>.[57]<\/a><\/sup>\u00a0Up to 90 percent of adults with depression are found to have sleep difficulties. Dysregulation detected by EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements.[58]<\/a><\/sup>\r\n\r\nSleep duration can also vary according to season. Up to 90% of people report longer sleep duration in winter, which may lead to more pronounced\u00a0seasonal affective disorder<\/a>.[59]<\/a><\/sup>[60]<\/a><\/sup>\r\n

Children<\/span><\/h3>\r\n
\r\n
\r\n\r\n\"\"<\/a>\r\n
\r\n\r\nBy the time infants reach the age of two, their brain size has reached 90 percent of an adult-sized brain;<\/span>[61]<\/a><\/sup>\u00a0a majority of this brain growth has occurred during the period of life with the highest rate of sleep. The hours that children spend asleep influence their ability to perform on cognitive tasks.<\/span>[62]<\/a><\/sup>[63]<\/a><\/sup>\u00a0Children who sleep through the night and have few night waking episodes have higher cognitive attainments and easier temperaments than other children.<\/span>[63]<\/a><\/sup>[64]<\/a><\/sup>[65]<\/a><\/sup>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nSleep also influences language development. To test this, researchers taught infants a faux language and observed their recollection of the rules for that language.[66]<\/a><\/sup>\u00a0Infants who slept within four hours of learning the language could remember the language rules better, while infants who stayed awake longer did not recall those rules as well. There is also a relationship between infants' vocabulary and sleeping: infants who sleep longer at night at 12 months have better vocabularies at 26 months.[65]<\/a><\/sup>\r\n

Recommendations<\/span><\/h3>\r\n
\r\n
\r\n\r\n\"\"<\/a>\r\n
\r\n\r\nChildren need many hours of sleep per day in order to develop and function properly: up to 18 hours for\u00a0<\/span>newborn<\/a>\u00a0babies, with a declining rate as a child ages.<\/span>[46]<\/a><\/sup>\u00a0Early in 2015, after a two-year study,<\/span>[67]<\/a><\/sup>\u00a0the\u00a0<\/span>National Sleep Foundation<\/a>\u00a0in the US announced newly-revised recommendations as shown in the table below.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Hours of sleep required for each age group[67]<\/a><\/sup><\/caption>\r\n
Age and condition<\/th>\r\nSleep needs<\/th>\r\n<\/tr>\r\n
Newborns (0\u20133 months)<\/td>\r\n14 to 17 hours<\/td>\r\n<\/tr>\r\n
Infants (4\u201311 months)<\/td>\r\n12 to 15 hours<\/td>\r\n<\/tr>\r\n
Toddlers (1\u20132 years)<\/td>\r\n11 to 14 hours<\/td>\r\n<\/tr>\r\n
Preschoolers (3\u20134 years)<\/td>\r\n10 to 13 hours<\/td>\r\n<\/tr>\r\n
School-age children (5\u201312 years)<\/td>\r\n9 to 11 hours<\/td>\r\n<\/tr>\r\n
Teenagers (13\u201317 years)<\/td>\r\n8 to 10 hours<\/td>\r\n<\/tr>\r\n
Adults (18\u201364 years)<\/td>\r\n7 to 9 hours<\/td>\r\n<\/tr>\r\n
Older Adults (65 years and over)<\/td>\r\n7 to 8 hours<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

Functions<\/span><\/h2>\r\n

Restoration<\/span><\/h3>\r\nThe human organism physically restores itself during sleep, healing itself, and removing metabolic wastes that build up during periods of activity. This restoration takes place mostly during\u00a0slow-wave sleep<\/a>, during which body temperature, heart rate, and brain oxygen consumption decrease. In both the brain and in the rest of the body, the reduced rate of\u00a0metabolism<\/a>\u00a0enables countervailing restorative processes.[68]<\/a><\/sup>\u00a0The brain, especially, requires sleep for restoration, whereas in the rest of the body these processes can take place during quiescent waking. Therefore, it is now thought that the essential function of sleep is its restorative effect on the brain.[69]<\/a><\/sup>\u00a0This theory is strengthened by the fact that sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least evolved animals which have no need for other functions of sleep such as memory consolidation or dreaming.[5]<\/a><\/sup>\r\n\r\nWhile awake, brain metabolism generates waste products such as\u00a0reactive oxygen species<\/a>, which are damaging to brain cells and inhibit their proper function. During sleep, metabolic rates decrease and reactive oxygen species generation is reduced allowing restorative processes to take over. The sleeping brain has been shown to remove metabolic waste products at a faster rate than during an awake state. This is accomplished via the\u00a0glymphatic system<\/a>.[70]<\/a><\/sup>\u00a0It is further theorized that sleep helps facilitate the synthesis of molecules that help repair and protect the brain from these harmful elements generated during waking.[71]<\/a><\/sup>\u00a0Anabolic hormones such as growth hormones are secreted preferentially during sleep. The concentration of the sugar compound\u00a0glycogen<\/a>\u00a0in the brain increases during sleep, and is depleted through metabolism during wakefulness.[68]<\/a><\/sup>\r\n\r\nStudies suggest that sleep deprivation may impair the body's ability to\u00a0heal wounds<\/a>.[72]<\/a><\/sup>\r\n\r\nIt has been shown that sleep deprivation affects the\u00a0immune system<\/a>\u00a0in rats.[73]<\/a><\/sup>\u00a0It is now possible to state that \"sleep loss impairs immune function and immune challenge alters sleep,\" and it has been suggested that sleep increases white blood cell counts.[74]<\/a><\/sup>\u00a0A 2014 study found that depriving mice of sleep increased cancer growth and dampened the immune system's ability to control cancers.[75]<\/a><\/sup>\r\n\r\nThe effect of sleep duration on\u00a0somatic<\/a>\u00a0growth is not completely known. One study recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1\u201310). It was found that \"the variation of sleep duration among children does not seem to have an effect on growth.\"[76]<\/a><\/sup>\u00a0It is well-established that\u00a0slow-wave sleep<\/a>\u00a0affects\u00a0growth hormone<\/a>\u00a0levels in adult men.[10]<\/a><\/sup>\u00a0During eight hours' sleep, Van Cauter, Leproult, and Plat found that the men with a high percentage of\u00a0slow-wave sleep (SWS)<\/a>\u00a0(average 24%) also had high growth hormone secretion, while subjects with a low percentage of\u00a0SWS<\/a>\u00a0(average 9%) had low growth hormone secretion.[77]<\/a><\/sup>\r\n

Memory processing<\/span><\/h3>\r\n
Further information:\u00a0Sleep and memory<\/a>,\u00a0Neuroscience of sleep<\/a>, and\u00a0Sleep and learning<\/a><\/div>\r\nIt has been widely accepted that sleep must support the formation of long-term memory, and generally increasing previous learning and experiences recalls. However, its benefit seems to depend on the phase of sleep and the type of memory.[78]<\/a><\/sup>\u00a0For example, declarative and procedural memory-recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep) does so.[79]<\/a><\/sup>[80]<\/a><\/sup>\r\n\r\nWith regard to declarative memory, the functional role of SWS has been associated with hippocampal replays of previously encoded neural patterns that seem to facilitate long-term memory consolidation.[79]<\/a><\/sup>[80]<\/a><\/sup>\u00a0This assumption is based on the active system consolidation hypothesis, which states that repeated reactivations of newly-encoded information in the hippocampus during slow oscillations in NREM sleep mediate the stabilization and gradual integration of declarative memory with pre-existing knowledge networks on the cortical level.[81]<\/a><\/sup>\u00a0It assumes the hippocampus might hold information only temporarily and in a fast-learning rate, whereas the neocortex is related to long-term storage and a slow-learning rate.[79]<\/a><\/sup>[80]<\/a><\/sup>[82]<\/a><\/sup>[83]<\/a><\/sup>[84]<\/a><\/sup>\u00a0This dialogue between the hippocampus and neocortex occurs in parallel with hippocampal\u00a0sharp-wave ripples<\/a>\u00a0and\u00a0thalamo-cortical spindles<\/a>, synchrony that drives the formation of the spindle-ripple event which seems to be a prerequisite for the formation of long-term memories.[80]<\/a><\/sup>[82]<\/a><\/sup>[84]<\/a><\/sup>[85]<\/a><\/sup>\r\n\r\nReactivation of memory also occurs during wakefulness and its function is associated with serving to update the reactivated memory with newly-encoded information, whereas reactivations during SWS are presented as crucial for memory stabilization.[80]<\/a><\/sup>\u00a0Based on targeted memory reactivation (TMR) experiments that use associated memory cues to triggering memory traces during sleep, several studies have been reassuring the importance of nocturnal reactivations for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people\u2019s memory performance at declarative recalls.[79]<\/a><\/sup>[83]<\/a><\/sup>[84]<\/a><\/sup>[85]<\/a><\/sup>[86]<\/a><\/sup>\r\n\r\nFurthermore, nocturnal reactivation seems to share the same neural oscillatory patterns as reactivation during wakefulness, processes which might be coordinated by\u00a0theta activity<\/a>.[87]<\/a><\/sup>\u00a0During wakefulness, theta oscillations have been often related to successful performance in memory tasks, and cued memory reactivations during sleep have been showing that theta activity is significantly stronger in subsequent recognition of cued stimuli as compared to uncued ones, possibly indicating a strengthening of memory traces and lexical integration by cuing during sleep.[88]<\/a><\/sup>\u00a0However, the beneficial effect of TMR for memory consolidation seems to occur only if the cued memories can be related to prior knowledge.[89]<\/a><\/sup>\r\n

Dreaming<\/span><\/h3>\r\n
<\/div>\r\n
\r\n
\r\n\r\n[caption id=\"\" align=\"alignleft\" width=\"220\"]\"\"<\/a> Dreams often feel like waking life, yet with added surrealism.[\/caption]\r\n\r\n
\r\n
During sleep, especially REM sleep, humans tend to experience dreams. These are elusive and mostly unpredictable first-person experiences which seem logical and realistic to the dreamer while they are in progress, despite their frequently bizarre, irrational, and\/or surreal qualities that become apparent when assessed after waking. Dreams often seamlessly incorporate concepts, situations, people, and objects within a person's mind that would not normally go together. They can include apparent sensations of all types, especially vision and movement.<\/span>[90]<\/a><\/sup><\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nDreams tend to rapidly fade from memory after waking. Some people choose to keep a\u00a0dream journal<\/a>, which they believe helps them build dream recall and facilitate the ability to experience\u00a0lucid dreams<\/a>.\r\n\r\nPeople have proposed many\u00a0hypotheses<\/a>\u00a0about the functions of dreaming.\u00a0Sigmund Freud<\/a>\u00a0postulated that dreams are the symbolic expression of frustrated desires that have been relegated to the\u00a0unconscious mind<\/a>, and he used\u00a0dream interpretation<\/a>\u00a0in the form of\u00a0psychoanalysis<\/a>\u00a0in attempting to uncover these desires.[91]<\/a><\/sup>\r\n\r\nCounterintuitively,\u00a0penile erections during sleep<\/a>\u00a0are not more frequent during sexual dreams than during other dreams.[92]<\/a><\/sup>\u00a0The parasympathetic nervous system experiences increased activity during REM sleep which may cause erection of the penis or clitoris. In males, 80% to 95% of REM sleep is normally accompanied by partial to full penile erection, while only about 12% of men's dreams contain sexual content.[93]<\/a><\/sup>\r\n\r\nJohn Allan Hobson<\/a>\u00a0and\u00a0Robert McCarley<\/a>\u00a0propose that dreams are caused by the random firing of neurons in the\u00a0cerebral cortex<\/a>\u00a0during the REM period. Neatly, this theory helps explain the irrationality of the mind during REM periods, as, according to this theory, the\u00a0forebrain<\/a>\u00a0then creates a\u00a0story<\/a>\u00a0in an attempt to reconcile and make sense of the nonsensical sensory information presented to it. This would explain the odd nature of many dreams.[94]<\/a><\/sup>\r\n\r\nUsing\u00a0antidepressants<\/a>,\u00a0acetaminophen<\/a>,\u00a0ibuprofen<\/a>, or\u00a0alcoholic beverages<\/a>\u00a0is thought to potentially suppress dreams, whereas\u00a0melatonin<\/a>\u00a0may have the ability to encourage them.[95]<\/a><\/sup>\r\n

Disorders<\/span><\/h2>\r\n

Insomnia<\/span><\/h3>\r\n
See also:\u00a0Psychological stress and sleep<\/a><\/div>\r\nInsomnia<\/a>\u00a0is a general term for difficulty falling asleep and\/or staying asleep. Insomnia is the most common sleep problem, with many adults reporting occasional insomnia, and 10\u201315% reporting a chronic condition.[96]<\/a><\/sup>\u00a0Insomnia can have many different causes, including\u00a0psychological stress<\/a>, a poor sleep environment, an inconsistent sleep schedule, or excessive mental or physical stimulation in the hours before bedtime. Insomnia is often treated through behavioral changes like keeping a regular sleep schedule, avoiding stimulating or stressful activities before bedtime, and cutting down on stimulants such as caffeine. The sleep environment may be improved by installing heavy drapes to shut out all sunlight, and keeping computers, televisions, and work materials out of the sleeping area.\r\n\r\nA 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise\u00a0may<\/i>\u00a0be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However, there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep.[97]<\/a><\/sup>\u00a0Sleeping medications such as\u00a0Ambien<\/a>\u00a0and\u00a0Lunesta<\/a>\u00a0are an increasingly popular treatment for insomnia. Although these nonbenzodiazepine medications are generally believed to be better and safer than earlier generations of sedatives, they have still generated some controversy and discussion regarding side effects.\u00a0White noise<\/a>\u00a0appears to be a promising treatment for\u00a0insomnia<\/a>.[98]<\/a><\/sup>\r\n

Obstructive sleep apnea<\/span><\/h3>\r\nObstructive sleep apnea<\/a>\u00a0is a condition in which major pauses in breathing occur during sleep, disrupting the normal progression of sleep and often causing other more severe health problems. Apneas occur when the muscles around the patient's airway relax during sleep, causing the airway to collapse and block the intake of oxygen.[99]<\/a><\/sup>\u00a0Obstructive sleep apnea is more common than\u00a0central sleep apnea<\/a>.[100]<\/a><\/sup>\u00a0As oxygen levels in the blood drop, the patient then comes out of deep sleep in order to resume breathing. When several of these episodes occur per hour, sleep apnea rises to a level of seriousness that may require treatment.\r\n\r\nDiagnosing sleep apnea usually requires a professional sleep study performed in a sleep clinic, because the episodes of wakefulness caused by the disorder are extremely brief and patients usually do not remember experiencing them. Instead, many patients simply feel tired after getting several hours of sleep and have no idea why. Major risk factors for sleep apnea include chronic fatigue, old age, obesity, and snoring.\r\n

Aging and sleep<\/span><\/h3>\r\nPeople over age 60 with prolonged sleep (8-10 hours or more; average sleep duration of 7-8 hours in the elderly) have a 33% increased risk of all-cause\u00a0mortality<\/a>\u00a0and 43% increased risk of\u00a0cardiovascular diseases<\/a>, while those with short sleep (less than 7 hours) have a 6% increased risk of all-cause mortality.[101]<\/a><\/sup>\u00a0Sleep disorders, including\u00a0sleep apnea<\/a>, insomnia, or periodic limb movements, occur more commonly in the elderly, each possibly impacting sleep quality and duration.[101]<\/a><\/sup>\u00a0A 2017 review indicated that older adults do not need less sleep, but rather have an impaired ability to obtain their sleep needs, and may be able to deal with sleepiness better than younger adults.[102]<\/a><\/sup>\u00a0Various practices are recommended to mitigate sleep disturbances in the elderly, such as having a light bedtime snack, avoidance of\u00a0caffeine<\/a>, daytime naps, excessive evening stimulation, and\u00a0tobacco<\/a>\u00a0products, and using regular bedtime and wake schedules.[103]<\/a><\/sup>\r\n

Other disorders<\/span><\/h3>\r\nSleep disorders include\u00a0narcolepsy<\/a>,\u00a0periodic limb movement disorder<\/a>\u00a0(PLMD),\u00a0restless leg syndrome<\/a>\u00a0(RLS),\u00a0upper airway resistance syndrome<\/a>\u00a0(UARS), and the circadian rhythm sleep disorders.\u00a0Fatal familial insomnia<\/a>, or FFI, an extremely rare genetic disease with no known treatment or cure, is characterized by increasing insomnia as one of its symptoms; ultimately sufferers of the disease stop sleeping entirely, before dying of the disease.[104]<\/a><\/sup>\r\n\r\nSomnambulism<\/a>, known as sleepwalking, is a sleeping disorder, especially among children.[105]<\/a><\/sup>\r\n

Sleep health<\/span><\/h2>\r\n
See also:\u00a0Sleep hygiene<\/a><\/div>\r\nLow quality sleep has been linked with health conditions like\u00a0cardiovascular disease<\/a>,\u00a0obesity<\/a>, and\u00a0mental illness<\/a>. While poor sleep is common among those with cardiovascular disease, some research indicates that poor sleep can be a contributing cause. Short sleep duration of less than seven hours is correlated with\u00a0coronary heart disease<\/a>\u00a0and increased risk of death from coronary heart disease. Sleep duration greater than nine hours is also correlated with coronary heart disease, as well as\u00a0stroke<\/a>\u00a0and cardiovascular events.[106]<\/a><\/sup>\r\n\r\nIn both children and adults, short sleep duration is associated with an increased risk of obesity, with various studies reporting an increased risk of 45\u201355%. Other aspects of sleep health have been associated with obesity, including daytime napping, sleep timing, the variability of sleep timing, and low sleep efficiency. However, sleep duration is the most-studied for its impact on obesity.[106]<\/a><\/sup>\r\n\r\nSleep problems have been frequently viewed as a symptom of mental illness rather than a causative factor. However, a growing body of evidence suggests that they are both a cause and a symptom of mental illness. Insomnia is a significant predictor of\u00a0major depressive disorder<\/a>; a\u00a0meta-analysis<\/a>\u00a0of 170,000 people showed that insomnia at the beginning of a study period indicated a more than the twofold increased risk for major depressive disorder. Some studies have also indicated correlation between insomnia and\u00a0anxiety<\/a>,\u00a0post-traumatic stress disorder<\/a>, and\u00a0suicide<\/a>. Sleep disorders can increase the risk of\u00a0psychosis<\/a>\u00a0and worsen the severity of psychotic episodes.[106]<\/a><\/sup>\r\n

Drugs and diet<\/span><\/h2>\r\nDrugs which induce sleep, known as\u00a0hypnotics<\/a>, include\u00a0benzodiazepines<\/a>, although these interfere with REM;[107]<\/a><\/sup>\u00a0Nonbenzodiazepine<\/a>\u00a0hypnotics such as\u00a0eszopiclone<\/a>\u00a0(Lunesta),\u00a0zaleplon<\/a>\u00a0(Sonata), and\u00a0zolpidem<\/a>\u00a0(Ambien);\u00a0Antihistamines<\/a>, such as\u00a0diphenhydramine<\/a>\u00a0(Benadryl) and\u00a0doxylamine<\/a>;\u00a0Alcohol (ethanol)<\/a>, despite its\u00a0rebound effect<\/a>\u00a0later in the night and interference with REM;[107]<\/a><\/sup>[108]<\/a><\/sup>\u00a0barbiturates<\/a>, which have the same problem;\u00a0melatonin<\/a>, a component of the circadian clock, and released naturally at night by the\u00a0pineal gland<\/a>;[109]<\/a><\/sup>\u00a0and\u00a0cannabis<\/a>, which may also interfere with REM.[110]<\/a><\/sup>\r\n\r\nStimulants<\/a>, which inhibit sleep, include\u00a0caffeine<\/a>, an\u00a0adenosine<\/a>\u00a0antagonist;\u00a0amphetamine<\/a>,\u00a0MDMA<\/a>,\u00a0empathogen-entactogens<\/a>, and related drugs;\u00a0cocaine<\/a>, which can alter the circadian rhythm,[111]<\/a><\/sup>[112]<\/a><\/sup>\u00a0and\u00a0methylphenidate<\/a>, which acts similarly; and other\u00a0analeptic<\/a>\u00a0drugs like\u00a0modafinil<\/a>\u00a0and\u00a0armodafinil<\/a>\u00a0with poorly understood mechanisms.\r\n\r\nDietary and nutritional choices may affect sleep duration and quality. One 2016 review indicated that a high-carbohydrate<\/a>\u00a0diet promoted a shorter onset to sleep and a longer duration of sleep than a high-fat diet.[113]<\/a><\/sup>\u00a0A 2012 investigation indicated that mixed\u00a0micronutrients<\/a>\u00a0and\u00a0macronutrients<\/a>\u00a0are needed to promote quality sleep.[114]<\/a><\/sup>\u00a0A varied diet containing fresh fruits and vegetables, low\u00a0saturated fat<\/a>, and\u00a0whole grains<\/a>\u00a0may be optimal for individuals seeking to improve sleep quality.[113]<\/a><\/sup>\u00a0High-quality\u00a0clinical trials<\/a>\u00a0on long-term dietary practices are needed to better define the influence of diet on sleep quality.[113]<\/a><\/sup>","rendered":"

Sleep<\/b>\u00a0is a naturally recurring state of mind and body, characterized by altered\u00a0consciousness, relatively inhibited sensory activity, reduced\u00a0muscle\u00a0activity and inhibition of nearly all\u00a0voluntary muscles\u00a0during\u00a0rapid eye<\/strong> movement\u00a0(REM) sleep,[1]<\/a>\u00a0and reduced interactions with surroundings.[2]<\/a>\u00a0It is distinguished from\u00a0wakefulness\u00a0by a decreased ability to react to\u00a0stimuli, but more reactive than a\u00a0coma\u00a0or\u00a0disorders of consciousness, with sleep displaying very different and active brain patterns.<\/p>\n

\"\"<\/p>\n

Sleep is associated with a state of muscle relaxation and reduced perception of environmental stimuli.<\/p>\n<\/div>\n

Sleep occurs in\u00a0repeating periods<\/a>, in which the body alternates between two distinct modes:\u00a0REM<\/a>\u00a0sleep and\u00a0non-REM<\/a>\u00a0sleep. Although REM stands for “rapid eye movement”, this mode of sleep has many other aspects, including virtual\u00a0paralysis<\/a>\u00a0of the body. A well-known feature of sleep is the\u00a0dream<\/a>, an experience typically recounted in\u00a0narrative<\/a>\u00a0form, which resembles waking life while in progress, but which usually can later be distinguished as fantasy. During sleep, most of the\u00a0body’s systems<\/a>\u00a0are in an\u00a0anabolic<\/a>\u00a0state, helping to restore the\u00a0immune<\/a>,\u00a0nervous<\/a>,\u00a0skeletal<\/a>, and\u00a0muscular<\/a>\u00a0systems; these are vital processes that maintain mood, memory, and cognitive function, and play a large role in the function of the\u00a0endocrine<\/a>\u00a0and\u00a0immune systems<\/a>.[3]<\/a><\/sup>\u00a0The internal\u00a0circadian clock<\/a>\u00a0promotes sleep daily at night. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research.[4]<\/a><\/sup>\u00a0Sleep is a highly\u00a0conserved<\/a>\u00a0behavior across animal evolution.[5]<\/a><\/sup><\/p>\n

Humans may suffer from various\u00a0sleep disorders<\/a>, including\u00a0dyssomnias<\/a>\u00a0such as\u00a0insomnia<\/a>,\u00a0hypersomnia<\/a>,\u00a0narcolepsy<\/a>, and\u00a0sleep apnea<\/a>;\u00a0parasomnias<\/a>\u00a0such as\u00a0sleepwalking<\/a>\u00a0and\u00a0rapid eye movement sleep behavior disorder<\/a>;\u00a0bruxism<\/a>; and\u00a0circadian rhythm sleep disorders<\/a>. The use of\u00a0artificial light<\/a>\u00a0has substantially altered humanity’s sleep patterns.[6]<\/a><\/sup><\/p>\n

Physiology<\/span><\/h2>\n
Main article:\u00a0Neuroscience of sleep<\/a><\/div>\n
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An artist’s creative illustration depicting REM sleep<\/p>\n<\/div>\n

\n
<\/div>\n
The most pronounced physiological changes in sleep occur in the brain.<\/span>[7]<\/a><\/sup>\u00a0The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of\u00a0<\/span>adenosine triphosphate<\/a>\u00a0(ATP), the molecule used for short-term storage and transport of energy.<\/span>[8]<\/a><\/sup>\u00a0In quiet waking, the brain is responsible for 20% of the body’s energy use, thus this reduction has a noticeable effect on overall energy consumption.<\/span>[9]<\/a><\/sup><\/div>\n<\/div>\n<\/div>\n<\/div>\n

Sleep increases the\u00a0sensory threshold<\/a>. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events.[9]<\/a><\/sup>[7]<\/a><\/sup><\/p>\n

During\u00a0slow-wave sleep<\/a>, humans secrete bursts of\u00a0growth hormone<\/a>. All sleep, even during the day, is associated with secretion of\u00a0prolactin<\/a>.[10]<\/a><\/sup><\/p>\n

Key physiological methods for monitoring and measuring changes during sleep include\u00a0electroencephalography<\/a>\u00a0(EEG) of\u00a0brain waves<\/a>,\u00a0electrooculography<\/a>\u00a0(EOG) of eye movements, and\u00a0electromyography<\/a>\u00a0(EMG) of\u00a0skeletal muscle<\/a>\u00a0activity. Simultaneous collection of these measurements is called\u00a0polysomnography<\/a>, and can be performed in a specialized\u00a0sleep laboratory<\/a>.[11]<\/a><\/sup>[12]<\/a><\/sup>\u00a0Sleep researchers also use simplified\u00a0electrocardiography<\/a>\u00a0(EKG) for cardiac activity and\u00a0actigraphy<\/a>\u00a0for motor movements.[12]<\/a><\/sup><\/p>\n

Non-REM and REM sleep<\/span><\/h3>\n

Sleep is divided into two broad types:\u00a0non-rapid eye movement<\/a>\u00a0(non-REM or NREM) sleep and\u00a0rapid eye movement<\/a>\u00a0(REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is called\u00a0slow-wave sleep<\/a>\u00a0or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy.[7]<\/a><\/sup>\u00a0REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for\u00a0dreams<\/a>\u00a0(or\u00a0nightmares<\/a>), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone,[2]<\/a><\/sup>\u00a0and suspension of\u00a0homeostasis<\/a>.[13]<\/a><\/sup><\/p>\n

The\u00a0sleep cycle<\/a>\u00a0of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4\u20136 times in a good night’s sleep.[12]<\/a><\/sup>[14]<\/a><\/sup>\u00a0The\u00a0American Academy of Sleep Medicine<\/a>\u00a0(AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or\u00a0slow-wave sleep<\/a>.[15]<\/a><\/sup>\u00a0The whole period normally proceeds in the order: N1 \u2192 N2 \u2192 N3 \u2192 N2 \u2192 REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep.[2]<\/a><\/sup>\u00a0There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.[12]<\/a><\/sup><\/p>\n

Awakening<\/span><\/h3>\n
<\/div>\n
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“The Awakening”, an illustration to writing by\u00a0Leo Tolstoy<\/p>\n<\/div>\n

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Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal\u00a0<\/span>circadian indicators<\/a>, along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle.<\/span>[16]<\/a><\/sup>\u00a0Awakening involves heightened electrical activation in the brain, beginning with the\u00a0<\/span>thalamus<\/a>\u00a0and spreading throughout the\u00a0<\/span>cortex<\/a>.<\/span>[16]<\/a><\/sup><\/div>\n<\/div>\n<\/div>\n<\/div>\n

During a night’s sleep, a small amount of time is usually spent in a waking state. As measured by electroencephalography, young females are awake for 0\u20131% of the larger sleeping period; young males are awake for 0\u20132%. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13\u201314% in the fifth. Most of this awake time occurred shortly after REM sleep.[16]<\/a><\/sup><\/p>\n

Today, many humans wake up with an\u00a0alarm clock<\/a>;[17]<\/a><\/sup>\u00a0however, people can also reliably wake themselves up at a specific time with no need for an alarm.[16]<\/a><\/sup>\u00a0Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization.[18]<\/a><\/sup>[17]<\/a><\/sup>\u00a0Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle.[19]<\/a><\/sup>[20]<\/a><\/sup>\u00a0Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying\u00a0sleep inertia<\/a>.[21]<\/a><\/sup><\/p>\n

Timing<\/span><\/h2>\n

Sleep timing is controlled by the\u00a0circadian clock<\/a>\u00a0(Process C), sleep-wake\u00a0homeostasis<\/a>\u00a0(Process S), and to some extent by the individual will.<\/p>\n

Circadian clock<\/span><\/h3>\n
Main article:\u00a0Circadian rhythm<\/a><\/div>\n
Further information:\u00a0Circadian rhythm sleep disorder<\/a><\/div>\n
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The human “biological clock”<\/p>\n<\/div>\n

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<\/div>\n

 <\/p>\n<\/div>\n<\/div>\n<\/div>\n

Sleep timing depends greatly on\u00a0hormonal<\/a>\u00a0signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism’s environment to recreate an internal day\u2013night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in\u00a0diurnal<\/a>\u00a0animals) and augments it at night.[22]<\/a><\/sup>[18]<\/a><\/sup>\u00a0The\u00a0suprachiasmatic nucleus<\/a>\u00a0(SCN), a brain area directly above the\u00a0optic chiasm<\/a>, is presently considered the most important nexus for this process; however, secondary clock systems have been found throughout the body.<\/p>\n

An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be\u00a0entrained<\/a><\/i>; an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period that slightly exceeds 24 hours. Scientists refer to such conditions as\u00a0free-running<\/a>\u00a0of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day.[17]<\/a><\/sup>[23]<\/a><\/sup>[24]<\/a><\/sup><\/p>\n

The circadian clock exerts constant influence on the body, affecting\u00a0sinusoidal<\/a>\u00a0oscillation of\u00a0body temperature<\/a>\u00a0between roughly 36.2\u00a0\u00b0C and 37.2\u00a0\u00b0C.[24]<\/a><\/sup>[25]<\/a><\/sup>\u00a0The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night.[26]<\/a><\/sup>\u00a0The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to the\u00a0pineal gland<\/a>, which releases the hormone\u00a0melatonin<\/a>\u00a0at night.[26]<\/a><\/sup>\u00a0Cortisol<\/a>\u00a0levels typically rise throughout the night,\u00a0peak in the awakening hours<\/a>, and diminish during the day.[10]<\/a><\/sup>[27]<\/a><\/sup>\u00a0Circadian\u00a0prolactin<\/a>\u00a0secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone.[10]<\/a><\/sup><\/p>\n

The circadian rhythm influences the ideal timing of a restorative sleep episode.[17]<\/a><\/sup>[28]<\/a><\/sup>\u00a0Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereas\u00a0slow-wave sleep<\/a>\u00a0can occur more independently of circadian time.[24]<\/a><\/sup><\/p>\n

The internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, at\u00a0the right moment<\/a>\u00a0in the circadian cycle, can significantly ‘reset’ the internal clock.[25]<\/a><\/sup>\u00a0Blue light, in particular, exerts the strongest effect,[18]<\/a><\/sup>\u00a0leading to concerns that\u00a0electronic media use<\/a>\u00a0before bed may interfere with sleep.[19]<\/a><\/sup><\/p>\n

Modern humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially\u00a0night shifts<\/a>), long-distance travel, and the influence of universal indoor lighting.[24]<\/a><\/sup>\u00a0Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely, they can have difficulty waking up in the trough of the cycle.[16]<\/a><\/sup>\u00a0A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise.[24]<\/a><\/sup><\/p>\n

Process S<\/span><\/h3>\n
Main article:\u00a0Sleep debt<\/a><\/div>\n

Generally speaking, the longer an organism is awake, the more it feels a need to sleep (“sleep debt”). This driver of sleep is referred to as\u00a0Process S<\/b>. The balance between sleeping and waking is regulated by a process called\u00a0homeostasis<\/a>. Induced or perceived lack of sleep is called\u00a0sleep deprivation<\/a>.<\/p>\n

Process S is driven by the depletion of\u00a0glycogen<\/a>\u00a0and accumulation of\u00a0adenosine<\/a>\u00a0in the forebrain that disinhibits the\u00a0ventrolateral preoptic nucleus<\/a>, allowing for inhibition of the\u00a0ascending reticular activating system<\/a>.[29]<\/a><\/sup><\/p>\n

Sleep deprivation tends to cause slower brain waves in the\u00a0frontal cortex<\/a>, shortened attention span, higher anxiety, impaired memory, and a grouchy mood. Conversely, a well-rested organism tends to have improved memory and mood.[30]<\/a><\/sup>\u00a0Neurophysiological and functional\u00a0imaging studies<\/a>\u00a0have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.[31]<\/a><\/sup><\/p>\n

There is disagreement on how much sleep debt is possible to accumulate, and whether sleep debt is accumulated against an individual’s average sleep or some other benchmark. It is also unclear whether the prevalence of sleep debt among adults has changed appreciably in the\u00a0industrialized world<\/a>\u00a0in recent decades. Sleep debt does show some evidence of being cumulative. Subjectively, however, humans seem to reach maximum sleepiness after 30 hours of waking up.[24]<\/a><\/sup>\u00a0It is likely that in\u00a0Western societies<\/a>, children are sleeping less than they previously have.[32]<\/a><\/sup><\/p>\n

One neurochemical indicator of sleep debt is\u00a0adenosine<\/a>, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine levels increase in the cortex and basal forebrain during prolonged wakefulness, and decrease during the sleep-recovery period, potentially acting as a homeostatic regulator of sleep.[33]<\/a><\/sup>[34]<\/a><\/sup>\u00a0Coffee<\/a>\u00a0and\u00a0caffeine<\/a>\u00a0temporarily block the effect of adenosine, prolong sleep latency, and reduce total sleep time and quality.[35]<\/a><\/sup><\/p>\n

Social timing<\/span><\/h3>\n

Humans are also influenced by aspects of\u00a0social time<\/i>, such as the hours when other people are awake, the hours when work is required, the time on the clock, etc.\u00a0Time zones<\/a>, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. The approximate nature of the time zone can be shown with China, a country which used to span five time zones and now\u00a0officially uses only one<\/a>\u00a0(UTC+8).[17]<\/a><\/sup><\/p>\n

Distribution<\/span><\/h3>\n

In\u00a0polyphasic sleep<\/a>, an organism sleeps several times in a 24-hour cycle, whereas in monophasic sleep this occurs all at once. Under experimental conditions, humans tend to alternate more frequently between sleep and wakefulness (i.e., exhibit more polyphasic sleep) if they have nothing better to do.[24]<\/a><\/sup>\u00a0Given a 14-hour period of darkness in experimental conditions, humans tended towards bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Bimodal sleep in humans was more common before the\u00a0industrial revolution<\/a>.[27]<\/a><\/sup><\/p>\n

Different characteristic sleep patterns, such as the familiarly so-called “early bird<\/a>” and “night owl<\/a>“, are called\u00a0chronotypes<\/a><\/i>. Genetics and sex have some influence on chronotype, but so do habits. Chronotype is also liable to change over the course of a person’s lifetime. Seven-year-olds are better disposed to wake up early in the morning than are fifteen-year-olds.[18]<\/a><\/sup>[17]<\/a><\/sup>\u00a0Chronotypes far outside the normal range are called circadian rhythm sleep disorders.[36]<\/a><\/sup><\/p>\n

Naps<\/span><\/h4>\n
<\/div>\n
\n
\n

\"\"<\/a><\/p>\n

\n

The\u00a0<\/span>siesta<\/a>\u00a0habit has recently been associated with a 37% lower coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep.<\/span>[37]<\/a><\/sup>\u00a0Short naps at mid-day and mild evening exercise were found to be effective for improved sleep, cognitive tasks, and mental health in elderly people.<\/span>[38]<\/a><\/sup><\/p>\n<\/div>\n<\/div>\n<\/div>\n

Genetics<\/span><\/h3>\n

Monozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Neurotransmitters, molecules whose production can be traced to specific genes, are one genetic influence on sleep that can be analyzed. The circadian clock has its own set of genes.[39]<\/a><\/sup>\u00a0Genes which may influence sleep include\u00a0ABCC9<\/a>,\u00a0DEC2<\/a>,\u00a0Dopamine receptor D2<\/a>[40]<\/a><\/sup>\u00a0and variants near\u00a0PAX 8<\/a>\u00a0and\u00a0VRK2<\/a>.[41]<\/a><\/sup><\/p>\n

Quality<\/span><\/h3>\n

The quality of sleep may be evaluated from an objective and a subjective point of view. Objective sleep quality refers to how difficult it is for a person to fall asleep and remain in a sleeping state, and how many times they wake up during a single night. Poor sleep quality disrupts the cycle of transition between the different stages of sleep.[42]<\/a><\/sup>\u00a0Subjective sleep quality in turn refers to a sense of being rested and regenerated after awaking from sleep. A study by A. Harvey et al. (2002) found that insomniacs were more demanding in their evaluations of sleep quality than individuals who had no sleep problems.[43]<\/a><\/sup><\/p>\n

Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep.[44]<\/a><\/sup>[45]<\/a><\/sup>\u00a0Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.[46]<\/a><\/sup>\u00a0The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:[47]<\/a><\/sup>\u00a0maximum concentration of the hormone melatonin, and minimum core body temperature.<\/p>\n

Ideal duration<\/span><\/h2>\n
\n
\n
\"\"<\/a><\/p>\n

The main health effects of\u00a0sleep deprivation, indicating impairment of normal maintenance by sleep<\/p>\n<\/div>\n

\n
<\/div>\n

Human sleep-needs vary by age and amongst individuals; sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by\u00a0<\/span>actigraphy<\/a>,<\/span>[49]<\/a><\/sup>\u00a0and those affected with\u00a0<\/span>sleep state misperception<\/a>\u00a0may typically report having slept only four hours despite having slept a full eight hours.<\/span>[50]<\/a><\/sup><\/p>\n<\/div>\n<\/div>\n<\/div>\n

Researchers have found that sleeping 6\u20137 hours each night correlates with longevity and cardiac health in humans, though many underlying factors may be involved in the causality behind this relationship.[51]<\/a><\/sup>[52]<\/a><\/sup>[53]<\/a><\/sup>[54]<\/a><\/sup>[41]<\/a><\/sup>[55]<\/a><\/sup>[56]<\/a><\/sup><\/p>\n

Sleep difficulties are furthermore associated with psychiatric disorders such as\u00a0depression<\/a>,\u00a0alcoholism<\/a>, and\u00a0bipolar disorder<\/a>.[57]<\/a><\/sup>\u00a0Up to 90 percent of adults with depression are found to have sleep difficulties. Dysregulation detected by EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements.[58]<\/a><\/sup><\/p>\n

Sleep duration can also vary according to season. Up to 90% of people report longer sleep duration in winter, which may lead to more pronounced\u00a0seasonal affective disorder<\/a>.[59]<\/a><\/sup>[60]<\/a><\/sup><\/p>\n

Children<\/span><\/h3>\n
\n
\n

\"\"<\/a><\/p>\n

\n

By the time infants reach the age of two, their brain size has reached 90 percent of an adult-sized brain;<\/span>[61]<\/a><\/sup>\u00a0a majority of this brain growth has occurred during the period of life with the highest rate of sleep. The hours that children spend asleep influence their ability to perform on cognitive tasks.<\/span>[62]<\/a><\/sup>[63]<\/a><\/sup>\u00a0Children who sleep through the night and have few night waking episodes have higher cognitive attainments and easier temperaments than other children.<\/span>[63]<\/a><\/sup>[64]<\/a><\/sup>[65]<\/a><\/sup><\/p>\n<\/div>\n<\/div>\n<\/div>\n

Sleep also influences language development. To test this, researchers taught infants a faux language and observed their recollection of the rules for that language.[66]<\/a><\/sup>\u00a0Infants who slept within four hours of learning the language could remember the language rules better, while infants who stayed awake longer did not recall those rules as well. There is also a relationship between infants’ vocabulary and sleeping: infants who sleep longer at night at 12 months have better vocabularies at 26 months.[65]<\/a><\/sup><\/p>\n

Recommendations<\/span><\/h3>\n
\n
\n

\"\"<\/a><\/p>\n

\n

Children need many hours of sleep per day in order to develop and function properly: up to 18 hours for\u00a0<\/span>newborn<\/a>\u00a0babies, with a declining rate as a child ages.<\/span>[46]<\/a><\/sup>\u00a0Early in 2015, after a two-year study,<\/span>[67]<\/a><\/sup>\u00a0the\u00a0<\/span>National Sleep Foundation<\/a>\u00a0in the US announced newly-revised recommendations as shown in the table below.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n\n\n\n\n\n\n\n\n
Hours of sleep required for each age group[67]<\/a><\/sup><\/caption>\n
Age and condition<\/th>\nSleep needs<\/th>\n<\/tr>\n
Newborns (0\u20133 months)<\/td>\n14 to 17 hours<\/td>\n<\/tr>\n
Infants (4\u201311 months)<\/td>\n12 to 15 hours<\/td>\n<\/tr>\n
Toddlers (1\u20132 years)<\/td>\n11 to 14 hours<\/td>\n<\/tr>\n
Preschoolers (3\u20134 years)<\/td>\n10 to 13 hours<\/td>\n<\/tr>\n
School-age children (5\u201312 years)<\/td>\n9 to 11 hours<\/td>\n<\/tr>\n
Teenagers (13\u201317 years)<\/td>\n8 to 10 hours<\/td>\n<\/tr>\n
Adults (18\u201364 years)<\/td>\n7 to 9 hours<\/td>\n<\/tr>\n
Older Adults (65 years and over)<\/td>\n7 to 8 hours<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Functions<\/span><\/h2>\n

Restoration<\/span><\/h3>\n

The human organism physically restores itself during sleep, healing itself, and removing metabolic wastes that build up during periods of activity. This restoration takes place mostly during\u00a0slow-wave sleep<\/a>, during which body temperature, heart rate, and brain oxygen consumption decrease. In both the brain and in the rest of the body, the reduced rate of\u00a0metabolism<\/a>\u00a0enables countervailing restorative processes.[68]<\/a><\/sup>\u00a0The brain, especially, requires sleep for restoration, whereas in the rest of the body these processes can take place during quiescent waking. Therefore, it is now thought that the essential function of sleep is its restorative effect on the brain.[69]<\/a><\/sup>\u00a0This theory is strengthened by the fact that sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least evolved animals which have no need for other functions of sleep such as memory consolidation or dreaming.[5]<\/a><\/sup><\/p>\n

While awake, brain metabolism generates waste products such as\u00a0reactive oxygen species<\/a>, which are damaging to brain cells and inhibit their proper function. During sleep, metabolic rates decrease and reactive oxygen species generation is reduced allowing restorative processes to take over. The sleeping brain has been shown to remove metabolic waste products at a faster rate than during an awake state. This is accomplished via the\u00a0glymphatic system<\/a>.[70]<\/a><\/sup>\u00a0It is further theorized that sleep helps facilitate the synthesis of molecules that help repair and protect the brain from these harmful elements generated during waking.[71]<\/a><\/sup>\u00a0Anabolic hormones such as growth hormones are secreted preferentially during sleep. The concentration of the sugar compound\u00a0glycogen<\/a>\u00a0in the brain increases during sleep, and is depleted through metabolism during wakefulness.[68]<\/a><\/sup><\/p>\n

Studies suggest that sleep deprivation may impair the body’s ability to\u00a0heal wounds<\/a>.[72]<\/a><\/sup><\/p>\n

It has been shown that sleep deprivation affects the\u00a0immune system<\/a>\u00a0in rats.[73]<\/a><\/sup>\u00a0It is now possible to state that “sleep loss impairs immune function and immune challenge alters sleep,” and it has been suggested that sleep increases white blood cell counts.[74]<\/a><\/sup>\u00a0A 2014 study found that depriving mice of sleep increased cancer growth and dampened the immune system’s ability to control cancers.[75]<\/a><\/sup><\/p>\n

The effect of sleep duration on\u00a0somatic<\/a>\u00a0growth is not completely known. One study recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1\u201310). It was found that “the variation of sleep duration among children does not seem to have an effect on growth.”[76]<\/a><\/sup>\u00a0It is well-established that\u00a0slow-wave sleep<\/a>\u00a0affects\u00a0growth hormone<\/a>\u00a0levels in adult men.[10]<\/a><\/sup>\u00a0During eight hours’ sleep, Van Cauter, Leproult, and Plat found that the men with a high percentage of\u00a0slow-wave sleep (SWS)<\/a>\u00a0(average 24%) also had high growth hormone secretion, while subjects with a low percentage of\u00a0SWS<\/a>\u00a0(average 9%) had low growth hormone secretion.[77]<\/a><\/sup><\/p>\n

Memory processing<\/span><\/h3>\n
Further information:\u00a0Sleep and memory<\/a>,\u00a0Neuroscience of sleep<\/a>, and\u00a0Sleep and learning<\/a><\/div>\n

It has been widely accepted that sleep must support the formation of long-term memory, and generally increasing previous learning and experiences recalls. However, its benefit seems to depend on the phase of sleep and the type of memory.[78]<\/a><\/sup>\u00a0For example, declarative and procedural memory-recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep) does so.[79]<\/a><\/sup>[80]<\/a><\/sup><\/p>\n

With regard to declarative memory, the functional role of SWS has been associated with hippocampal replays of previously encoded neural patterns that seem to facilitate long-term memory consolidation.[79]<\/a><\/sup>[80]<\/a><\/sup>\u00a0This assumption is based on the active system consolidation hypothesis, which states that repeated reactivations of newly-encoded information in the hippocampus during slow oscillations in NREM sleep mediate the stabilization and gradual integration of declarative memory with pre-existing knowledge networks on the cortical level.[81]<\/a><\/sup>\u00a0It assumes the hippocampus might hold information only temporarily and in a fast-learning rate, whereas the neocortex is related to long-term storage and a slow-learning rate.[79]<\/a><\/sup>[80]<\/a><\/sup>[82]<\/a><\/sup>[83]<\/a><\/sup>[84]<\/a><\/sup>\u00a0This dialogue between the hippocampus and neocortex occurs in parallel with hippocampal\u00a0sharp-wave ripples<\/a>\u00a0and\u00a0thalamo-cortical spindles<\/a>, synchrony that drives the formation of the spindle-ripple event which seems to be a prerequisite for the formation of long-term memories.[80]<\/a><\/sup>[82]<\/a><\/sup>[84]<\/a><\/sup>[85]<\/a><\/sup><\/p>\n

Reactivation of memory also occurs during wakefulness and its function is associated with serving to update the reactivated memory with newly-encoded information, whereas reactivations during SWS are presented as crucial for memory stabilization.[80]<\/a><\/sup>\u00a0Based on targeted memory reactivation (TMR) experiments that use associated memory cues to triggering memory traces during sleep, several studies have been reassuring the importance of nocturnal reactivations for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people\u2019s memory performance at declarative recalls.[79]<\/a><\/sup>[83]<\/a><\/sup>[84]<\/a><\/sup>[85]<\/a><\/sup>[86]<\/a><\/sup><\/p>\n

Furthermore, nocturnal reactivation seems to share the same neural oscillatory patterns as reactivation during wakefulness, processes which might be coordinated by\u00a0theta activity<\/a>.[87]<\/a><\/sup>\u00a0During wakefulness, theta oscillations have been often related to successful performance in memory tasks, and cued memory reactivations during sleep have been showing that theta activity is significantly stronger in subsequent recognition of cued stimuli as compared to uncued ones, possibly indicating a strengthening of memory traces and lexical integration by cuing during sleep.[88]<\/a><\/sup>\u00a0However, the beneficial effect of TMR for memory consolidation seems to occur only if the cued memories can be related to prior knowledge.[89]<\/a><\/sup><\/p>\n

Dreaming<\/span><\/h3>\n
<\/div>\n
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\n
\"\"<\/a><\/p>\n

Dreams often feel like waking life, yet with added surrealism.<\/p>\n<\/div>\n

\n
During sleep, especially REM sleep, humans tend to experience dreams. These are elusive and mostly unpredictable first-person experiences which seem logical and realistic to the dreamer while they are in progress, despite their frequently bizarre, irrational, and\/or surreal qualities that become apparent when assessed after waking. Dreams often seamlessly incorporate concepts, situations, people, and objects within a person’s mind that would not normally go together. They can include apparent sensations of all types, especially vision and movement.<\/span>[90]<\/a><\/sup><\/div>\n<\/div>\n<\/div>\n<\/div>\n

Dreams tend to rapidly fade from memory after waking. Some people choose to keep a\u00a0dream journal<\/a>, which they believe helps them build dream recall and facilitate the ability to experience\u00a0lucid dreams<\/a>.<\/p>\n

People have proposed many\u00a0hypotheses<\/a>\u00a0about the functions of dreaming.\u00a0Sigmund Freud<\/a>\u00a0postulated that dreams are the symbolic expression of frustrated desires that have been relegated to the\u00a0unconscious mind<\/a>, and he used\u00a0dream interpretation<\/a>\u00a0in the form of\u00a0psychoanalysis<\/a>\u00a0in attempting to uncover these desires.[91]<\/a><\/sup><\/p>\n

Counterintuitively,\u00a0penile erections during sleep<\/a>\u00a0are not more frequent during sexual dreams than during other dreams.[92]<\/a><\/sup>\u00a0The parasympathetic nervous system experiences increased activity during REM sleep which may cause erection of the penis or clitoris. In males, 80% to 95% of REM sleep is normally accompanied by partial to full penile erection, while only about 12% of men’s dreams contain sexual content.[93]<\/a><\/sup><\/p>\n

John Allan Hobson<\/a>\u00a0and\u00a0Robert McCarley<\/a>\u00a0propose that dreams are caused by the random firing of neurons in the\u00a0cerebral cortex<\/a>\u00a0during the REM period. Neatly, this theory helps explain the irrationality of the mind during REM periods, as, according to this theory, the\u00a0forebrain<\/a>\u00a0then creates a\u00a0story<\/a>\u00a0in an attempt to reconcile and make sense of the nonsensical sensory information presented to it. This would explain the odd nature of many dreams.[94]<\/a><\/sup><\/p>\n

Using\u00a0antidepressants<\/a>,\u00a0acetaminophen<\/a>,\u00a0ibuprofen<\/a>, or\u00a0alcoholic beverages<\/a>\u00a0is thought to potentially suppress dreams, whereas\u00a0melatonin<\/a>\u00a0may have the ability to encourage them.[95]<\/a><\/sup><\/p>\n

Disorders<\/span><\/h2>\n

Insomnia<\/span><\/h3>\n
See also:\u00a0Psychological stress and sleep<\/a><\/div>\n

Insomnia<\/a>\u00a0is a general term for difficulty falling asleep and\/or staying asleep. Insomnia is the most common sleep problem, with many adults reporting occasional insomnia, and 10\u201315% reporting a chronic condition.[96]<\/a><\/sup>\u00a0Insomnia can have many different causes, including\u00a0psychological stress<\/a>, a poor sleep environment, an inconsistent sleep schedule, or excessive mental or physical stimulation in the hours before bedtime. Insomnia is often treated through behavioral changes like keeping a regular sleep schedule, avoiding stimulating or stressful activities before bedtime, and cutting down on stimulants such as caffeine. The sleep environment may be improved by installing heavy drapes to shut out all sunlight, and keeping computers, televisions, and work materials out of the sleeping area.<\/p>\n

A 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise\u00a0may<\/i>\u00a0be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However, there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep.[97]<\/a><\/sup>\u00a0Sleeping medications such as\u00a0Ambien<\/a>\u00a0and\u00a0Lunesta<\/a>\u00a0are an increasingly popular treatment for insomnia. Although these nonbenzodiazepine medications are generally believed to be better and safer than earlier generations of sedatives, they have still generated some controversy and discussion regarding side effects.\u00a0White noise<\/a>\u00a0appears to be a promising treatment for\u00a0insomnia<\/a>.[98]<\/a><\/sup><\/p>\n

Obstructive sleep apnea<\/span><\/h3>\n

Obstructive sleep apnea<\/a>\u00a0is a condition in which major pauses in breathing occur during sleep, disrupting the normal progression of sleep and often causing other more severe health problems. Apneas occur when the muscles around the patient’s airway relax during sleep, causing the airway to collapse and block the intake of oxygen.[99]<\/a><\/sup>\u00a0Obstructive sleep apnea is more common than\u00a0central sleep apnea<\/a>.[100]<\/a><\/sup>\u00a0As oxygen levels in the blood drop, the patient then comes out of deep sleep in order to resume breathing. When several of these episodes occur per hour, sleep apnea rises to a level of seriousness that may require treatment.<\/p>\n

Diagnosing sleep apnea usually requires a professional sleep study performed in a sleep clinic, because the episodes of wakefulness caused by the disorder are extremely brief and patients usually do not remember experiencing them. Instead, many patients simply feel tired after getting several hours of sleep and have no idea why. Major risk factors for sleep apnea include chronic fatigue, old age, obesity, and snoring.<\/p>\n

Aging and sleep<\/span><\/h3>\n

People over age 60 with prolonged sleep (8-10 hours or more; average sleep duration of 7-8 hours in the elderly) have a 33% increased risk of all-cause\u00a0mortality<\/a>\u00a0and 43% increased risk of\u00a0cardiovascular diseases<\/a>, while those with short sleep (less than 7 hours) have a 6% increased risk of all-cause mortality.[101]<\/a><\/sup>\u00a0Sleep disorders, including\u00a0sleep apnea<\/a>, insomnia, or periodic limb movements, occur more commonly in the elderly, each possibly impacting sleep quality and duration.[101]<\/a><\/sup>\u00a0A 2017 review indicated that older adults do not need less sleep, but rather have an impaired ability to obtain their sleep needs, and may be able to deal with sleepiness better than younger adults.[102]<\/a><\/sup>\u00a0Various practices are recommended to mitigate sleep disturbances in the elderly, such as having a light bedtime snack, avoidance of\u00a0caffeine<\/a>, daytime naps, excessive evening stimulation, and\u00a0tobacco<\/a>\u00a0products, and using regular bedtime and wake schedules.[103]<\/a><\/sup><\/p>\n

Other disorders<\/span><\/h3>\n

Sleep disorders include\u00a0narcolepsy<\/a>,\u00a0periodic limb movement disorder<\/a>\u00a0(PLMD),\u00a0restless leg syndrome<\/a>\u00a0(RLS),\u00a0upper airway resistance syndrome<\/a>\u00a0(UARS), and the circadian rhythm sleep disorders.\u00a0Fatal familial insomnia<\/a>, or FFI, an extremely rare genetic disease with no known treatment or cure, is characterized by increasing insomnia as one of its symptoms; ultimately sufferers of the disease stop sleeping entirely, before dying of the disease.[104]<\/a><\/sup><\/p>\n

Somnambulism<\/a>, known as sleepwalking, is a sleeping disorder, especially among children.[105]<\/a><\/sup><\/p>\n

Sleep health<\/span><\/h2>\n
See also:\u00a0Sleep hygiene<\/a><\/div>\n

Low quality sleep has been linked with health conditions like\u00a0cardiovascular disease<\/a>,\u00a0obesity<\/a>, and\u00a0mental illness<\/a>. While poor sleep is common among those with cardiovascular disease, some research indicates that poor sleep can be a contributing cause. Short sleep duration of less than seven hours is correlated with\u00a0coronary heart disease<\/a>\u00a0and increased risk of death from coronary heart disease. Sleep duration greater than nine hours is also correlated with coronary heart disease, as well as\u00a0stroke<\/a>\u00a0and cardiovascular events.[106]<\/a><\/sup><\/p>\n

In both children and adults, short sleep duration is associated with an increased risk of obesity, with various studies reporting an increased risk of 45\u201355%. Other aspects of sleep health have been associated with obesity, including daytime napping, sleep timing, the variability of sleep timing, and low sleep efficiency. However, sleep duration is the most-studied for its impact on obesity.[106]<\/a><\/sup><\/p>\n

Sleep problems have been frequently viewed as a symptom of mental illness rather than a causative factor. However, a growing body of evidence suggests that they are both a cause and a symptom of mental illness. Insomnia is a significant predictor of\u00a0major depressive disorder<\/a>; a\u00a0meta-analysis<\/a>\u00a0of 170,000 people showed that insomnia at the beginning of a study period indicated a more than the twofold increased risk for major depressive disorder. Some studies have also indicated correlation between insomnia and\u00a0anxiety<\/a>,\u00a0post-traumatic stress disorder<\/a>, and\u00a0suicide<\/a>. Sleep disorders can increase the risk of\u00a0psychosis<\/a>\u00a0and worsen the severity of psychotic episodes.[106]<\/a><\/sup><\/p>\n

Drugs and diet<\/span><\/h2>\n

Drugs which induce sleep, known as\u00a0hypnotics<\/a>, include\u00a0benzodiazepines<\/a>, although these interfere with REM;[107]<\/a><\/sup>\u00a0Nonbenzodiazepine<\/a>\u00a0hypnotics such as\u00a0eszopiclone<\/a>\u00a0(Lunesta),\u00a0zaleplon<\/a>\u00a0(Sonata), and\u00a0zolpidem<\/a>\u00a0(Ambien);\u00a0Antihistamines<\/a>, such as\u00a0diphenhydramine<\/a>\u00a0(Benadryl) and\u00a0doxylamine<\/a>;\u00a0Alcohol (ethanol)<\/a>, despite its\u00a0rebound effect<\/a>\u00a0later in the night and interference with REM;[107]<\/a><\/sup>[108]<\/a><\/sup>\u00a0barbiturates<\/a>, which have the same problem;\u00a0melatonin<\/a>, a component of the circadian clock, and released naturally at night by the\u00a0pineal gland<\/a>;[109]<\/a><\/sup>\u00a0and\u00a0cannabis<\/a>, which may also interfere with REM.[110]<\/a><\/sup><\/p>\n

Stimulants<\/a>, which inhibit sleep, include\u00a0caffeine<\/a>, an\u00a0adenosine<\/a>\u00a0antagonist;\u00a0amphetamine<\/a>,\u00a0MDMA<\/a>,\u00a0empathogen-entactogens<\/a>, and related drugs;\u00a0cocaine<\/a>, which can alter the circadian rhythm,[111]<\/a><\/sup>[112]<\/a><\/sup>\u00a0and\u00a0methylphenidate<\/a>, which acts similarly; and other\u00a0analeptic<\/a>\u00a0drugs like\u00a0modafinil<\/a>\u00a0and\u00a0armodafinil<\/a>\u00a0with poorly understood mechanisms.<\/p>\n

Dietary and nutritional choices may affect sleep duration and quality. One 2016 review indicated that a high-carbohydrate<\/a>\u00a0diet promoted a shorter onset to sleep and a longer duration of sleep than a high-fat diet.[113]<\/a><\/sup>\u00a0A 2012 investigation indicated that mixed\u00a0micronutrients<\/a>\u00a0and\u00a0macronutrients<\/a>\u00a0are needed to promote quality sleep.[114]<\/a><\/sup>\u00a0A varied diet containing fresh fruits and vegetables, low\u00a0saturated fat<\/a>, and\u00a0whole grains<\/a>\u00a0may be optimal for individuals seeking to improve sleep quality.[113]<\/a><\/sup>\u00a0High-quality\u00a0clinical trials<\/a>\u00a0on long-term dietary practices are needed to better define the influence of diet on sleep quality.[113]<\/a><\/sup><\/p>\n\n\t\t\t

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Public domain content<\/div>