Introduction to Sleep
Most adults need seven to nine hours of sleep to recuperate from physical exertion, as well as to consolidate learning and memory.
Summarize the various functions of sleep
- Sleep is essential for proper psychological and physiological functioning. Research on rats has found that a week of no sleep leads to loss of immune function, and two weeks of no sleep leads to death.
- Sleep has been linked to psychological, emotional, and mental recovery, as well as learning and memory. Recent research shows that sleep is vital in consolidating memories, both intellectual and physical.
- Sleep deprivation can inhibit your productivity and your ability to remember and consolidate information. It can also lead to serious health consequences such as increased risk of heart disease and obesity.
- The amount of sleep we need varies depending on multiple factors: age, physical condition, psychological condition, or energy exerted. Therefore, sleep requirements depend on the individual.
- REM sleep: The stage of sleep during which most brain activity and dreams occur, characterized by rapid eye movement (REM).
Why Do We Need Sleep?
Along with food and water, sleep is one of the human body’s most important physiological needs—we cannot live without it. Extended sleeplessness (i.e., lack of sleep for longer than a few days) has severe psychological and physical effects. Research on rats has found that a week of no sleep leads to loss of immune function, and two weeks of no sleep leads to death.
Despite its clear importance to psychological and physiological functioning, researchers have struggled for centuries to answer the question of why we sleep. On the surface, we know that sleep helps our body recuperate from a day’s physical exertions. It also aids in recovery from illnesses and infections. We also know that extended sleeplessness can lead to hallucinations, delusions, loss of immune function, and in extreme cases, death. Modern research has also uncovered that sleep has a major role in maintaining our mental and emotional health.
Recently, neuroscientists have learned that at least one vital function of sleep is related to learning and memory. New findings suggest that sleep plays a critical role in flagging and storing important memories, both intellectual and physical, and perhaps in making subtle connections that were invisible during waking hours.
Though scientists are still learning about the concept of basal sleep need (just how much sleep we need), research has demonstrated that sleeping too little can inhibit your productivity and your ability to remember and consolidate information. Lack of sleep can also lead to serious health consequences, jeopardizing individual safety and the safety of others.
For example, sleep deprivation is related to:
- higher rates of motor vehicle accidents;
- higher BMI, an increased likelihood of obesity, and increased risk of diabetes and heart problems;
- higher risk for depression and substance abuse;
- decreased attention, slower reaction times, and the inability to remember new information.
When we do not sleep enough, we accumulate a sleep debt. Sleep debt occurs as the result of not getting enough sleep, and a large debt causes mental, emotional, and physical fatigue. Sleep debt results in diminished abilities to perform high-level cognitive functions.
How Much Sleep Do We Need?
The amount of sleep we need varies depending on multiple factors including age, physical condition, psychological condition, and energy exertion. Therefore, sleep requirements depend entirely on the individual. Just like any other human characteristic, the amount of sleep people need to function best differs among individuals, even those of the same age and gender. For example, a pregnant woman will need more sleep than a healthy woman of the same age, an adult with a cold will need more sleep than one who is well, and an individual with depression may require more sleep than a non-depressed person.
Though there is no magic sleep number, there are general rules for how much sleep certain age groups need. For instance, children need more sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. A newborn baby spends almost 9 hours a day in REM sleep. By the age of five, only slightly over two hours is spent in REM. Studies show that young children need about 10 to 11 hours of sleep, adolescents need between 8.5 and 9.25, and adults generally need between 7 and 9 hours.
Effects of Sleep Deprivation
Research has uncovered a number of ways that a lack of sleep affects our well-being. Sleep deprivation negatively affects brain chemistry, growth, healing, attention, memory, and the ability to operate machinery, among other things. Sleep deprivation can cause both physical and mental illness, such as diabetes, depression, and psychosis, and in extreme cases, it can cause hallucinations and death.
Sleep Deprivation on the Brain and Body
Numerous studies have demonstrated that sleep deprivation can adversely affect brain growth and cognitive functions. fMRI studies performed on sleep-deprived subjects show that regions of the brain’s prefrontal cortex, an area that supports mental faculties such as working memory and logical reasoning, displayed more activity in sleepier subjects. The results implied that sleepier subjects had to work harder than well-rested subjects to accomplish the same task, indicating the need to compensate for adverse effects caused by sleep deprivation.
The link between sleep deprivation and psychosis has been well-documented. In 2007, a study at Harvard Medical School and the University of California at Berkeley revealed, using MRI scans, that sleep deprivation causes the brain to become incapable of putting an emotional event into the proper perspective and incapable of making a controlled, suitable response to the event.
The negative effects of sleep deprivation on alertness and cognitive performance suggest decreases in brain activity and function, primarily in the thalamus, a structure involved in alertness and attention, and in the prefrontal cortex, a region sub-serving alertness, attention, and higher-order cognitive processes.
Sleep deprivation has also been found to inhibit stress reactions, body functions such as digestion, the immune system, mood, sex, and energy usage, while also suppressing growth hormones. Sleep deprivation has also been shown to slow the healing process, and has been implicated in weight gain and type-2 diabetes.
Circadian rhythms are physical, mental, and behavioral changes that respond to light and darkness and are important in determining human sleep patterns.
Analyze the role that circadian rhythms play in the sleep cycle
- Circadian rhythms are important in determining human sleep patterns. The body’s master clock, or the suprachiasmatic nucleus (SCN), controls the production of melatonin, a hormone that makes a person sleepy.
- Since it is located just above the optic nerves, which relay information from the eyes to the brain, the SCN receives information about incoming light. When there is less light—such as at night—the SCN tells the brain to make more melatonin so a person gets drowsy.
- The circadian rhythm rises and falls at different times of the day, making us feel more alert or sleepy at certain points in the day. This may vary by a few hours depending on whether a person is naturally a “morning person” or a “night person.”
- chronobiology: The study of the effects of time on biological systems, especially the effects of periodicity.
- circadian: Of, relating to, or showing rhythmic behavior with a period of 24 hours; especially of a biological process.
- entrainment: The alignment of an organism’s circadian rhythm to that of an external rhythm in its environment.
Circadian rhythms are physical, mental, and behavioral changes that follow an approximate 24-hour cycle, responding primarily to light and darkness in an organism’s environment. Circadian rhythms are found in most living things, including animals, plants, and many tiny microbes. These rhythms can influence sleep-wake cycles, hormone release, body temperature, and other important bodily functions. The study of circadian rhythms (and biological temporal rhythms in general) is called chronobiology.
Circadian Rhythms and the Suprachiasmatic Nucleus
Circadian rhythms are important in determining human sleep patterns. The body’s master clock, or the suprachiasmatic nucleus (SCN), controls the production of melatonin, a hormone that makes you sleepy. The SCN is a small group of brain cells located in the hypothalamus that controls the circadian cycles and influences many physiological and behavioral rhythms occurring over a 24-hour period, including the sleep/wake cycle. The circadian pacemaker, located in the SCN, regulates the timing and consolidation of the sleep-wake cycle, while sleep-wake homeostasis governs the accumulation of sleep debt and sleep recovery. Destruction of the SCN results in the complete absence of a regular sleep-wake rhythm.
The Effect of Light on Circadian Rhythms
Circadian rhythms are maintained by individual organisms, and their length varies somewhat between individuals. Therefore, they must be reset, either continually or repeatedly, to synchronize with nature’s cycle. In order to maintain synchronization (a.k.a. entrainment ) to a 24-hour cycle, external factors must play some role; of these factors, light exposure to the eyes is the strongest.
The SCN receives information about light through the eyes. The retina of the eye contains photoreceptors (rods for seeing at night, and cones for distinguishing colors), which are used for conventional vision. But the retina also contains specialized ganglion cells, which are photosensitive and project directly to the SCN, where they help in the entrainment of this master circadian clock.
The mechanisms by which light affects entrainment are not yet fully known. However, studies have shown that timing of light exposure, the length of light exposure, and the intensity/wavelength of light all influence entrainment and circadian rhythms.
Circadian Rhythms and Sleep
Our internal circadian pacemakers regulate the timing and duration of sleepiness and wakefulness throughout the day. The circadian rhythm falls and rises at different times of the day; in fact, numerous studies have demonstrated that human circadian rhythms in many measures of performance and physiological activity actually have a 2-peak daily (circasemidian) pattern. A typical adult will experience the strongest desire to sleep in the pre-dawn hours (around 2:00 a.m. to 4:00 a.m.) and in the early afternoon (around 1:00 p.m. to 3:00 p.m.). This frame may be slightly different depending on whether you are a “morning person” or a “night person.” The sleepiness we experience during these circadian dips will be less intense if we have had sufficient sleep, and more intense when we are sleep deprived. The circadian rhythm also causes us to feel more alert at certain points of the day, even if we have been awake for hours.
When we have been awake for an extended period of time, sleep-wake homeostasis tells us that the need for sleep is accumulating, and staying awake becomes more difficult.
Segmented sleep, also known as interrupted or divided sleep, is a multiphasic sleep pattern in which two or more periods of sleep are punctuated by periods of wakefulness. Along with a nap at mid-day, it has been argued that this is the natural pattern of human sleep. In fact, some scientists believe that maintenance of this sleep pattern is important in regulating stress.
Roger Ekirch, a historian who has researched segmented sleep extensively, argues that segmented sleep
was the dominant form of human sleep before the Industrial Revolution. Circadian rhythms, as we’ve just discussed, are heavily influenced by light. Ekirch suggests that it is due to the modern use of electric lighting (computers, cell phones, office lights) that most modern humans do not practice
In one experiment on segmented sleep, researchers had eight healthy men confined to a room for fourteen hours of darkness every day for a month. At first the participants slept for about eleven hours, presumably
making up for their sleep debt. After that the subjects began to sleep much as people in pre-industrial times did. They would sleep for about four hours, wake up for two to three hours, then go back to bed for another four hours. They also took about two hours to fall asleep. Together, these results imply that segmented sleep is indeed our natural sleep rhythm.
Stages of Sleep
Sleep proceeds through multiple cycles of non-REM (3 stages) and REM per night, with each full cycle lasting between 90 and 110 minutes.
Differentiate among REM sleep and the three phases of N-REM sleep
- The overall sleep cycle is comprised of two broad alternating cycles, REM (rapid eye movement), and NREM (non-rapid eye movement), which consists of three individual phases—N1, N2, and N3.
- Stage 1 NREM is the “drifting off” stage, characterized by lowered brain activity, blood pressure, and muscle tone. During this 10-minute period, a sleeper may experience twitches and may even deny having slept at all.
- Stage 2 NREM is characterized by sleep spindles and K-complexes. During this stage, muscular activity decreases, and conscious awareness of the external environment disappears.
- Stage 3 NREM (formerly considered two stages, 3 and 4) is the deepest stage of NREM sleep. It is characterized by delta waves, and is also called slow-wave sleep or SWS. It occurs 30–45 minutes after falling asleep, and many environmental stimuli no longer produce any reactions.
- REM sleep is characterized by partial paralysis, vivid dreaming, and an EEG that resembles waking brain activity. It is also referred to as paradoxical sleep because the sleeper, although exhibiting waking-state EEG waves, is harder to arouse than at any other sleep stage.
- Electroencephalography: The recording of electrical activity along the scalp; it measures voltage fluctuations resulting from ionic current flows within neurons.
- Parasomnia: Category of sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams that occur during sleep.
In mammals, the overall sleep cycle is comprised of two broad alternating cycles, REM (rapid eye movement), and NREM (non-rapid eye movement), which itself consists of three individual phases. (Note: REM is pronounced like a word [“rehm”], not as a series of initials. NREM, or non-REM, is pronounced phonetically as well, with the “N” standing alone [“en-rehm”].)
In electroencephalography, the REM phase of sleep is easily identified by its paradigmatic fast, small-amplitude waves. (Also typical are the rapid eye movements for which the REM phase is named.) For this reason REM sleep is sometimes also called “active sleep.” Many sleep experts think that these eye movements are in some way related to dreams. Sleep proceeds in cycles of REM and NREM, usually four or five per night. These cycles typically last between 90 and 110 minutes.
Stages of NREM Sleep
Stage 1 NREM (N1)
After a person falls asleep, he or she enters into what is known as stage 1 of non-REM sleep. This stage is sometimes referred to as somnolence, or drowsy sleep. Sudden twitches known as hypnic jerks are associated with this stage. The sleeper loses some muscle tone and most consciousness of the external environment. This stage lasts around ten minutes, during which a person can still be woken easily. A sleeper who has experienced only stage 1 NREM sleep may not even realize they slept at all. Stage 1 NREM is characterized by:
- slow, even breathing,
- regular heartbeat;
- lowered brain temperature.
Stage 2 NREM (N2)
Stage 2 non-REM sleep is characterized by sleep spindles and K-Complexes.
A sleep spindle is a burst of oscillatory brain activity visible on an EEG that occurs specifically during stage 2 sleep. Sleep spindles are generated in the reticular nucleus of the thalamus, and may represent periods in which the brain is inhibiting processing to keep the sleeper in a tranquil state. During sleep these spindles are seen in the brain as a burst of activity immediately following muscle twitching. Spindles have been shown to aid sleeping in the presence of disruptive external sounds, and have also been associated with the integration of new information into existing knowledge, as well as with directed remembering and forgetting.
K-complexes are high-voltage events that occur more frequently in the first sleep cycles. K-complexes are thought to have two main purposes:
- to limit brain activity in response to stimuli that are not dangerous, and
- to help with sleep-based memory consolidation.
During this stage, muscular activity decreases, and conscious awareness of the external environment disappears. This stage occupies 45–55% of total sleep in adults. It is also characterized by:
- high-amplitude brain waves;
- sporadic short periods of increased activity;
- heightened sensitivity to sound stimuli (sleeper may wake up);
- slowed bodily functions (e.g., lower blood pressure; decreased cardiac and metabolic activity).
Stage 3 NREM (N3)
Stage 3 of non-REM sleep is considered the start to “deep sleep.” A sleeper first enters stage three approximately 30 to 45 minutes into sleeping. This is the stage in which different forms of parasomnia, such as night terrors, nocturnal enuresis, sleepwalking, and somniloquy (sleep talking) typically occur. This stage, formerly divided into two separate stages (3 and 4), is called slow-wave sleep, or SWS. In SWS, the sleeper is less responsive to the environment; many environmental stimuli no longer produce any reactions.
It is characterized by the following:
- Delta waves, or, very slow brain waves.
- Deeper sleep; it is much harder to wake up someone in this phase of sleep than someone in stage 1 or 2 sleep.
- If awakened during this phase, a person will likely experience disorientation and grogginess.
The sleeper will go through NREM stage 2 once again before entering REM sleep.
The sleeper now enters rapid eye movement (REM) sleep. In this stage, most muscles are paralyzed. This level is also referred to as paradoxical sleep because the sleeper, although exhibiting EEG waves similar to a waking state, is harder to arouse than at any other sleep stage.
REM sleep accounts for 20–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. During REM sleep, EEG patterns return to higher-frequency saw-tooth waves. Most memorable dreaming occurs in this stage. Partial paralysis occurs, perhaps to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage. Vital signs indicate arousal, and oxygen consumption by the brain is higher than when the sleeper is awake.
Sleep has four distinct phases, or stages. Sleep progresses from stage 1 to stage 2 to stage 3, and then back to stage 2 before transitioning into the REM phase. Once the REM phase is over, stage 2 will repeat. A person will complete this entire cycle about four or five times given a full night of sleep.
A sleeper first enters REM sleep after about an hour and a half of sleep, and then the phase will last only briefly. However, for each successive sleep cycle, the proportion of the cycle spent in REM sleep increases, up to an hour long in later cycles. This is why it is important for humans to sleep in long stretches (e.g., once a day for 8 hours) rather than in short bursts (e.g., 4 times a day for 2 hours each time); we spend much less total time in REM sleep than we do when we get one long period of sleep, and we can’t get through the stages we need to heal and stay healthy.
Sleep deprivation tends to cause slower brain waves in the frontal cortex, shortened attention span, higher anxiety, impaired memory, and an unhappy mood. Conversely, a well-rested organism tends to have improved memory and mood. Research has demonstrated that some sleep stages are more important than others in achieving restfulness. For example, REM deprivation causes a significant increase in the number of attempts to go into the REM stage while asleep. On recovery nights, an individual will most likely move to stage 3 and REM sleep more quickly and experience “REM rebound,” which refers to a large increase in the time spent in the REM stage. These findings are consistent with the idea that REM sleep is biologically necessary. REM sleep is thought to be largely responsible for the time that our brains take to consolidate learning and memory. Similarly, sleeping in short bursts is not sufficient for restfulness because it does not allow our bodies enough time for a complete cycle of sleep.
Sleep disorders cause sleep disturbances that affect the amount, quality, or timing of sleep or that induce abnormal events during sleep.
Compare the symptoms and causes of major sleep disorders
- Sleep-wake disorders can interfere with normal physical, mental, and emotional functioning.
- The major sleep-wake disorders are insomnia disorder, hypersomnolence disorder, and narcolepsy.
- There is also a category of sleep-wake disorders called “breathing-related sleep disorders,” which includes sleep apnea.
- Parasomnias are a category of sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, while sleeping, while between sleep stages, or during arousal from sleep.
- polysomnography: A measurement technique that documents an individual’s multiple different physiological procedures during the night, generally used for the purpose of diagnosing sleep-related disorders.
Sleep-wake disorders cause a number of sleep disturbances that affect the amount, quality, or timing of sleep or that induce abnormal events during sleep. Some sleep-wake disorders are serious enough to interfere with normal physical, mental, and emotional functioning. Disruptions in sleep can be caused by a variety of issues, from teeth grinding (bruxism) to night terrors. Polysomnography is a technique that can be used to identify some sleep disorders.
Insomnia refers to a chronic difficulty in falling asleep and/or maintaining sleep when no other cause is found for these symptoms. It is often a symptom of a mood disorder (e.g., emotional stress, anxiety, depression), an underlying health condition (e.g., asthma, diabetes, heart disease, pregnancy, or a neurological condition), or abuse of alcohol or drugs. However, it can also be a disorder in its own right.
The DSM-5 diagnostic criteria for insomnia disorder are as follows:
- The person experiences dissatisfaction with sleep quantity or quality as a result of difficulty initiating or maintaining sleep.
- As a result the person experiences clinically significant distress or impairment in social, occupational, educational, academic, behavioral, or other important areas of functioning.
- The sleep difficulty occurs despite adequate opportunity for sleep.
- The sleep difficulty occurs at least 3 nights per week.
- The sleep difficulty is present for at least 3 months.
The DSM-5 also provides the following exclusion criteria:
- The insomnia must not be better explained by and must not occur exclusively during the course of another sleep-wake disorder (e.g., narcolepsy, a breathing-related sleep disorder, a circadian rhythm sleep-wake disorder, a parasomnia).
- The insomnia must not be attributable to the physiological effects of a substance (e.g., a drug of abuse, a medication).
- Coexisting mental disorders and medical conditions must not adequately explain the predominant complaint of insomnia.
Hypersomnolence disorder (also known as idiopathic hypersomnia) is a disease of likely neurological origin that is characterized primarily by severe, excessive daytime sleepiness. It has been diagnosed only rarely and is often very difficult to diagnose at an early stage. It is usually a debilitating lifelong disease. There is a very low level of public awareness of this disorder, which often results in stigmatization of those who suffer from it. Currently, there is no cure, there are no FDA-approved treatments, and research funding for the study of this disorder is scarce.
Narcolepsy is also referred to as excessive daytime sleepiness (EDS). Individuals with this disorder often fall asleep spontaneously but unwillingly at inappropriate times. People with narcolepsy tend to have trouble in areas such as work, leisure, and personal relationships.
Breathing-Related Sleep Disorders
Some sleep disorders are caused by disturbances in breathing. Sleep apnea, for example, is a disorder in which obstruction of the airway during sleep causes a lack of sufficient deep sleep, often accompanied by snoring.
Sleep apnea is usually caused by some sort of physical condition that obstructs the breathing system, such as obesity, rather than a mental condition. The DSM-5 names two types of sleep apnea: obstructive sleep apnea, in which your airway collapses during sleep, and central sleep apnea, in which your brain does not signal your lungs to continue to inhale.
Individuals with sleep apnea often feel tired throughout the day, as the constant breaks in their sleep cycle cause unrestful nights.
Parasomnias are a category of sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, while sleeping, while between sleep stages, or during arousal from sleep. Most parasomnias are due to partial arousal during the transitions between wakefulness and non-rapid-eye-movement (N-REM) sleep or between wakefulness and rapid-eye-movement (REM) sleep.
Sleepwalking (sometimes called sleepwalking disorder, somnambulism, or noctambulation) causes a person to get up and walk during the early hours of sleep. The person may sit up and look awake (though they’re actually asleep), get up and walk around, move items, or dress or undress themselves. They will have a blank stare and still be able to perform complex tasks. Some individuals also talk while in their sleep, saying meaningless words and even having arguments with people who are not there. A person who sleepwalks will be confused upon waking up and may also experience anxiety and fatigue.
Sleepwalking can be dangerous—people have been known to seriously hurt themselves during sleepwalking episodes. It is most common in children, but it also occurs occasionally in adults. For adults, alcohol, sedatives, medications, medical conditions and mental disorders are all associated with sleepwalking.
Sleep Terrors and Nightmare Disorder
Sleep terrors are characterized by a sudden arousal from deep sleep with a scream or cry, accompanied by some behavioral manifestations of intense fear. Sleep terrors typically occur in the first few hours of sleep, during stage 3 NREM sleep. Night terrors tend to happen during periods of arousal from delta sleep (i.e., slow-wave sleep). They are worse than nightmares, causing significant disorientation, panic, and anxiety. They can last up to 10 minutes, and the person may be screaming and difficult to wake. In adults, it is often a symptom of some psychopathology.
Distinct from sleep terrors is nightmare disorder. Also known as “dream anxiety disorder,” nightmare disorder is characterized by frequent nightmares. The nightmares, which often portray the individual in a situation that jeopardizes their life or personal safety, usually occur during the second half of the sleeping process, called the REM stage. Though many people experience nightmares, those with nightmare disorder experience them more frequently.
Restless Legs Syndrome
Restless legs syndrome (RLS) is a neurological disorder characterized by an irresistible urge to move one’s body to stop uncomfortable or odd sensations. It most commonly affects the legs, but it can also affect the arms, torso, head, and even phantom limbs. Moving the affected body part modulates the sensations, providing temporary relief. RLS may start at any age, including childhood. For some it is a progressive disease; for others the symptoms decrease over time.
The Nature and Meaning of Dreams
Numerous theories, both psychological and neurobiological, have been proposed to explain the elusive mystery of the purpose of dreaming.
Explain the major theories surrounding the phenomenon of dreaming
- Sigmund Freud and Carl Jung put forth some of the best-known modern theories of dreaming; Freud believed that dreaming allows us to sort through unresolved, repressed wishes, while Jung believed that dreams present the dreamer with revelations to resolve emotional or religious problems and fears.
- Freud’s theory describes dreams as having both latent and manifest content; latent content relates to deep unconscious wishes or fantasies, while manifest content is superficial and meaningless.
- Threat-simulation theory suggests that dreaming should be seen as an ancient biological defense mechanism that provides an evolutionary advantage because of its capacity to repeatedly simulate potential threatening events, thus enhancing the mechanisms required for efficient threat avoidance.
- Activation-synthesis theory states that dreams don’t actually mean anything. Instead, dreams are merely electrical brain impulses that pull random thoughts and imagery from our memories. The theory posits that humans construct dream stories after they wake up, in an attempt to make sense of it all.
- The continual-activation theory proposes that dreaming is a result of brain activation and synthesis; its assumption is that, during REM sleep, the unconscious part of the brain is busy processing procedural memory.
- manifest: Obvious to the understanding; apparent to the mind; easily apprehensible; plain; not obscure or hidden.
- latent: Existing or present but concealed or inactive.
For centuries people have pondered the meaning of dreams. Early civilizations thought of dreams as a medium between our earthly world and that of the gods. In fact, the Greeks and Romans were convinced that dreams had certain prophetic powers. Over the years, numerous theories have been put forth in an attempt to illuminate the mystery behind human dreams. Only recently has strong, tangible evidence become less elusive.
Psychological Theories of Dreaming
Freudian and Jungian Theories
While there has always been great interest in the interpretation of human dreams, it was not until the end of the nineteenth century that Sigmund Freud and Carl Jung put forth some of the most widely-known modern theories of dreaming. Freud’s theory centered around the notion of repressed longing or wish fulfillment—the idea that dreaming allows us to sort through unresolved, repressed wishes. Freud’s theory described dreams as having both latent and manifest content. Latent content consists of deep unconscious wishes or fantasies, while manifest content is superficial and meaningless. Manifest content often masks or obscures latent content. It was in his book The Interpretation of Dreams (published in 1900) that Freud first argued that the motivation of all dream content is wish-fulfillment, and that the instigation of a dream is often to be found in the events of the day preceding the dream, which he called the “day residue.” Later, Freud revised his theory to suggest that dreams may also represent the repetition compulsion, which is a psychological phenomenon in which a person repeats a traumatic event or its circumstances over and over again. This can take the form of reenacting the event—which can include “reliving” the scenario in the form of dreams—or putting oneself in situations where the event is likely to happen again.
Carl Jung (who studied under Freud) also believed that dreams had psychological importance, but proposed different theories about their meaning. Jung expanded on Freud’s idea that dream content relates to the dreamer’s unconscious desires. He thought of dreams as messages to the dreamer, containing revelations that could uncover and possibly resolve emotional or religious problems and fears.
Threat-simulation theory suggests that dreaming should be seen as an ancient biological defense mechanism. Dreams are thought to provide an evolutionary advantage because of their capacity to repeatedly simulate potential threatening events. This process enhances the neurocognitive mechanisms required for efficient threat perception and avoidance.
During much of human evolution, physical and interpersonal threats were serious enough to reward reproductive advantage to those who survived them. Therefore, dreaming evolved to replicate these threats and allow people to regularly practice dealing with them. This theory suggests that dreams serve the purpose of allowing for the rehearsal of threatening scenarios in order to better prepare an individual for real-life threats.
This theory posits that dreaming serves to discharge emotional arousals (however minor) that haven’t been expressed during the day. This practice frees up space in the brain to deal with the emotional arousals of the next day and allows instinctive urges to stay intact. In effect, the expectation is fulfilled (the action is “completed”) in a metaphorical form so that a false memory is not created. This theory explains why dreams are usually forgotten immediately afterwards.
One prominent neurobiological theory of dreaming is the activation-synthesis theory, which states that dreams don’t actually mean anything. They are merely electrical brain impulses that pull random thoughts and imagery from our memories. The theory posits that humans construct dream stories after they wake up, in a natural attempt to make sense of the nonsensical. However, given the vast documentation of the realistic aspects of human dreaming, as well as indirect experimental evidence that other mammals such as cats also dream, evolutionary psychologists have theorized that dreaming does indeed serve a purpose.
The continual-activation theory proposes that dreaming is a result of brain activation and synthesis. Dreaming and REM sleep are simultaneously controlled by different brain mechanisms. The hypothesis states that the function of sleep is to process, encode, and transfer data from short-term memory to long-term memory through a process called consolidation. However, there is not much evidence to back this up. NREM sleep processes the conscious-related memory (declarative memory), and REM sleep processes the unconscious related memory (procedural memory).
The underlying assumption of continual-activation theory is that, during REM sleep, the unconscious part of the brain is busy processing procedural memory. Meanwhile, the level of activation in the conscious part of the brain descends to a very low level as the inputs from the senses are basically disconnected. This triggers the “continual-activation” mechanism to generate a data stream from the memory stores to flow through to the conscious part of the brain.