Sleep is a period of rest that alternates with wakefulness. You have internal body clocks that control when you are awake and when your body is ready for sleep. These clocks have cycles of approximately 24 hours. The clocks are regulated by multiple factors, including light, darkness, and sleep schedules. Once asleep, you cycle through the stages of sleep throughout the night in a predictable pattern. These sleep patterns change with age.

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. 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.

Many factors play a role in preparing your body to fall asleep and wake up. Your body has several internal clocks, called circadian clocks. These typically follow a 24-hour repeating rhythm, called the circadian rhythm. This rhythm affects every cell, tissue, and organ in your body and how they work. Your central circadian clock, located in your brain, tells you when it is time for sleep. Other circadian clocks are located in organs throughout your body. Your body’s internal clocks are in sync with certain cues in the environment. Light, darkness, and other cues help determine when you feel awake and when you feel drowsy. Artificial light and caffeine can disrupt this process by giving your body false wakefulness cues.

Your body has a biological need for sleep that increases when you have been awake for a long time. This is controlled by homeostasis, the process by which your body keeps your systems, such as your internal body temperature, steady. A compound called adenosine is linked to this need for sleep. While you are awake, the level of adenosine in your brain continues to rise. The rising levels signal a shift toward sleep. Caffeine and certain drugs can interrupt this process by blocking adenosine.

If you follow a natural schedule of days and nights, light signals received through your eyes tell your brain that it is daytime. The area of your brain that receives these signals, called the suprachiasmatic nucleus, transmits the signals to the rest of your body through the sympathetic system and the parasympathetic system. This helps your central body clock stay in tune with the day and night. Exposure to bright artificial light in the late evening can disrupt this process and prevent your brain from releasing melatonin. This can make it harder to fall asleep. Examples of bright artificial light include the light from a TV screen, a smartphone, or a very bright alarm clock.

The light–dark cycle influences when your brain makes and releases a hormone called melatonin. Melatonin travels to the cells in your body through your bloodstream. The amount of melatonin in your bloodstream starts to increase in the evening and peaks in the early morning. Melatonin is thought to promote sleep. As you are exposed to more light, such as the sun rising, your body releases another hormone called cortisol. Cortisol naturally prepares your body to wake up.

When you sleep, you cycle through two phases of sleep: rapid eye movement (REM) and non-REM sleep. The cycle starts over every 80 to 100 minutes. Usually, there are four to six cycles per night. You may wake up briefly between cycles. Sleep studies use sensors to record eye movements and brain activity, which are used to classify sleep phases and stages.

During REM sleep, your eyes twitch and your brain is active. Brain activity measured during REM sleep is similar to your brain’s activity during waking hours. Dreaming usually happens during REM sleep. Your muscles normally become limp to prevent you from acting out your dreams. You usually have more REM sleep later in the night, but you do not have as much REM sleep in colder temperatures. This is because, during REM sleep, your body does not regulate its temperature properly.

The patterns and types of sleep change as people mature. For example, newborns spend more time in REM sleep. The amount of slow-wave sleep peaks in early childhood and then drops sharply in the teenage years. Slow-wave sleep continues to decrease through adulthood, and older people may not have any slow-wave sleep at all.

The amount of sleep an individual needs varies depending on multiple factors including age, physical condition, psychological condition, and energy exertion. 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.

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.

Table 1. Typical amount of sleep by age

Infant Sleep

Infants 0 to 2 years of age sleep an average of 12.8 hours a day, although this changes and develops gradually throughout an infant’s life. For the first three months, newborns sleep between 14 and 17 hours a day, then they become increasingly alert for longer periods of time. About one-half of an infant’s sleep is rapid eye movement (REM) sleep, and infants often begin their sleep cycle with REM rather than non-REM sleep. They also move through the sleep cycle more quickly than adults. Parents spend a significant amount of time worrying about and losing even more sleep over their infant’s sleep schedule when there remains a great deal of variation in sleep patterns and habits for individual children. A 2018 study showed that at 6 months of age, 62% of infants slept at least six hours during the night, 43% of infants slept at least 8 hours through the night, and 38% of infants were not sleeping at least six continual hours through the night. At 12 months, 28% of children were still not sleeping at least 6 uninterrupted hours through the night, while 78% were sleeping at least 6 hours, and 56% were sleeping at least 8 hours.

The most common infant sleep-related problem reported by parents is nighttime waking. Studies of new parents and sleep patterns show that parents lose the most sleep during the first three months with a new baby, with mothers losing about an hour of sleep each night, and fathers losing a disproportionate 13 minutes. This decline in sleep quality and quantity for adults persists until the child is about six years old.

While this shows there is no precise science as to when and how an infant will sleep, there are general trends in sleep patterns. Around six months, babies typically sleep between 14-15 hours a day, with 3-4 of those hours happening during daytime naps. As they get older, these naps decrease from several to typically two naps a day between ages 9-18 months. Often, periods of rapid weight gain or changes in developmental abilities such as crawling or walking will cause changes to sleep habits as well. Infants generally move towards one 2-4 hour nap a day by around 18 months, and many children will continue to nap until around four or five years old.

Sudden Unexpected Infant Deaths (SUID)

Each year in the United States, there are about 3,500 Sudden Unexpected Infant Deaths (SUID). These deaths occur among infants less than one-year-old and have no immediately obvious cause (CDC, 2015). The three commonly reported types of SUID are:

• Sudden Infant Death Syndrome (SIDS): SIDS is identified when the death of a healthy infant occurs suddenly and unexpectedly, and medical and forensic investigation findings (including an autopsy) are inconclusive. SIDS is the leading cause of death in infants up to 12 months old, and approximately 1,500 infants died of SIDS in 2013 (CDC, 2015). The risk of SIDS is highest at 4 to 6 weeks of age. Because SIDS is diagnosed when no other cause of death can be determined, possible causes of SIDS are regularly researched. One leading hypothesis suggests that infants who die from SIDS have abnormalities in the area of the brainstem responsible for regulating breathing (Weekes-Shackelford & Shackelford, 2005). Although the exact cause is unknown, doctors have identified the following risk factors for SIDS:
  • low birth weight
  • siblings who have had SIDS
  • sleep apnea
  • of African-American or Eskimo decent
  • low socioeconomic status (SES)
  • smoking in the home
• Unknown Cause: The sudden death of an infant less than one year of age that cannot be explained because a thorough investigation was not conducted and the cause of death could not be determined.
• Accidental Suffocation and Strangulation in Bed: Reasons for accidental suffocation include the following: Suffocation by soft bedding, another person rolling on top of or against the infant while sleeping, an infant being wedged between two objects such as a mattress and wall, and strangulation such as when an infant’s head and neck become caught between crib railings.

The combined SUID rate declined considerably following the release of the American Academy of Pediatrics safe sleep recommendations in 1992, which advocated that infants be placed on their backs for sleep (non-prone position). These recommendations were followed by a major Back to Sleep Campaign in 1994. According to the CDC, the SIDS death rate is now less than one-fourth of what is was (130 per 100,000 live birth in 1990 versus 40 in 2015). However, accidental suffocation and strangulation in bed mortality rates remained unchanged until the late 1990s. Some parents were still putting newborns to sleep on their stomachs partly because of past tradition. Most SIDS victims experience several risks, an interaction of biological and social circumstances. But thanks to research, the major risk, stomach sleeping, has been highly publicized. Other causes of death during infancy include congenital birth defects and homicide.

Co-Sleeping

The location of sleep depends primarily on the baby’s age and culture. Bed-sharing (in the parents’ bed) or co-sleeping (in the parents’ room) is the norm is some cultures, but not in others (Esposito et al. 2015). Colvin, Collie-Akers, Schunn, and Moon (2014) analyzed a total of 8,207 deaths from 24 states during 2004–2012. The deaths were documented in the National Center for the Review and Prevention of Child Deaths Case Reporting System, a database of death reports from state child death review teams. The results indicated that younger victims (0-3 months) were more likely to die by bed-sharing and sleeping in an adult’s bed or on a person. A higher percentage of older victims (4 months to 364 days) rolled into objects in the sleep environment and changed position from side/back to prone. Carpenter et al. (2013) compared infants who died of SIDS with a matched control and found that infants younger than three months old who slept in bed with a parent were five times more likely to die of SIDS compared to babies who slept separately from the parents, but were still in the same room. They concluded that bed-sharing, even when the parents do not smoke or take alcohol or drugs, increases the risk of SIDS. However, when combined with parental smoking and maternal alcohol consumption and/or drug use, the risks associated with bed-sharing greatly increased.

Despite the risks noted above, the controversy about where babies should sleep has been ongoing. Co-sleeping has been recommended for those who advocate attachment parenting (Sears & Sears, 2001), and other research suggests that bed-sharing and co-sleeping is becoming more popular in the United States (Colson et al., 2013). So, what are the latest recommendations?

The American Academy of Pediatrics (AAP) actually updated their recommendations for a Safe Infant Sleeping Environment in 2016. The most recent AAP recommendations on creating a safe sleep environment include:

• Back to sleep for every sleep. Always place the baby on his or her back on a firm sleep surface such as a crib or bassinet with a tight-fitting sheet.
• Avoid the use of soft bedding, including crib bumpers, blankets, pillows, and soft toys. The crib should be bare.
• Breastfeeding is recommended.
• Share a bedroom with parents, but not the same sleeping surface, preferably until the baby turns 1 but at least for the first six months. Room-sharing decreases the risk of SIDS by as much as 50 percent.
• Avoid baby’s exposure to smoke, alcohol, and illicit drugs.

As you can see, there is a recommendation to now “share a bedroom with parents,” but not the same sleeping surface. Breastfeeding is also recommended as adding protection against SIDS, but after feeding, the AAP encourages parents to move the baby to his or her separate sleeping space, preferably a crib or bassinet in the parents’ bedroom. Finally, the report included new evidence that supports skin-to-skin care for newborn infants.

Sleep in Childhood

The American Academy of Sleep Medicine (2021) has recommended that toddlers get 11 to 14 hours of sleep in a 24 hour period. This may include naps throughout the day. Preschool children will naturally need fewer naps and will typically forgo naps by elementary school. This age group should sleep 10 to 13 hours per day. Children aged 6–12 years should regularly sleep 9 to 12 hours per 24 hours.

Children who do not get enough sleep have a higher risk of obesity, diabetes, injuries, poor mental health, and problems with attention and behavior.  Children who get the recommended hours of sleep on a regular basis tend to experience improved attention, behavior, learning, memory, emotional regulation, quality of life, and mental and physical health (American Academy of Sleep Medicine, 2021).

Sleep Hygiene

Good sleep habits (sometimes referred to as “sleep hygiene”) can help you get a good night’s sleep. This means having both a bedroom environment and daily routines that promote consistent, uninterrupted sleep.

Some habits that can improve sleep health:

• Be consistent. Go to bed at the same time each night and get up at the same time each morning, including on the weekends.
• Bedtime routines can signal to the body and brain that it is time to sleep. For children, this might include a bath, story, or songs before going to bed. Additionally, limit children’s time spent awake in bed doing non-sleep activities to avoid associating bed with play.
• Make sure the bedroom is quiet, dark, relaxing, and at a comfortable temperature, and avoid allowing children to fall asleep somewhere other than their bed.
• Remove electronic devices, such as TVs, computers, and smartphones, from the bedroom and avoid artificial light for an hour before bed.
• Avoid large meals, liquids two hours before, and caffeine eight hours before bedtime.
• Get some exercise. Being physically active during the day can help you fall asleep more easily at night.

Sleep Disturbances

Sometimes children have difficulty falling or staying asleep. Changes in routine, excitement, biological changes, health issues, or sleep disorders can be the culprit.

Here are a few suggestions to help children that are having sleep difficulties:

• Use relaxation techniques, such as deep breathing or imagining positive scenes, to help your child relax before bed.
• For children who worry a lot at bedtime, try scheduling a “worry time” earlier in the day and encourage them to talk about their worries with a parent.
• Security objects, such as a blanket or stuffed toy, can help children feel safe and secure at bedtime.
• If your child wakes up and has trouble falling back asleep, it is better for them to get out of bed. Do a quiet activity, such as reading, for 20-30 minutes. Then have them return to bed.

Sleep in Adolescence

Adolescents’ normal sleep patterns are different from those of children and adults. Teens are often drowsy upon waking, tired during the day, and wakeful at night. Although it may seem like teens are lazy, science shows that melatonin levels (or the “sleep hormone” levels) in the blood naturally rise later at night and fall later in the morning in teens than in most children and adults. This may explain why many teens stay up late and struggle with getting up in the morning.

According to the National Sleep Foundation (NSF) (2016), adolescents need about 8 to 10 hours of sleep each night to function best. The most recent Sleep in America poll in 2006 indicated that adolescents between sixth and twelfth grade were not getting the recommended amount of sleep. On average, adolescents only received 7 ½ hours of sleep per night on school nights with younger adolescents getting more than older ones (8.4 hours for sixth graders and only 6.9 hours for those in twelfth grade). For older adolescents, only about one in ten (9%) get an optimal amount of sleep, and they are more likely to experience negative consequences the following day. These include feeling too tired or sleepy, being cranky or irritable, falling asleep in school, having a depressed mood, and drinking caffeinated beverages (NSF, 2016). Additionally, they are at risk for substance abuse, car crashes, poor academic performance, obesity, and a weakened immune system (Weintraub, 2016).

Why don’t adolescents get adequate sleep? In addition to known environmental and social factors, including work, homework, media, technology, and socializing, the adolescent brain is also a factor. As adolescents go through puberty, their circadian rhythms change and push back their sleep time until later in the evening (Weintraub, 2016). This biological change not only keeps adolescents awake at night, but it also makes it difficult for them to get up in the morning. When they are awake too early, their brains do not function optimally. Impairments are noted in attention, behavior, and academic achievement, while increases in tardiness and absenteeism are also demonstrated.

To support adolescents’ later sleeping schedule, the Centers for Disease Control and Prevention recommended that school not begin any earlier than 8:30 a.m. Unfortunately, over 80% of American schools begin their day earlier than 8:30 a.m. with an average start time of 8:03 a.m. (Weintraub, 2016). Psychologists and other professionals have been advocating for later school times, and they have produced research demonstrating better student outcomes for later start times. More middle and high schools have changed their start times to reflect the sleep research better. However, the logistics of changing start times and bus schedules are proving too difficult for some schools leaving many adolescents vulnerable to the negative consequences of sleep deprivation.