Sleep

verting delet

For other uses, see Sleep (disambiguation).

"Waking up" redirects here. For other uses, see Waking Up (disambiguation).

Sleep is the natural state of bodily rest observed in humans and other animals. It is common to all mammals and birds, and is also seen in many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals that have been studied (such as some species of fish, birds, ants, and fruit flies), regular sleep is essential for survival.

A widely publicized 2003 study performed at the University of Pennsylvania School of Medicine provided evidence that cognitive performance in humans declines with fewer than eight hours of sleep. However, the purposes of sleep are only partly clear and are the subject of intense research.

Two men sleeping in Tehran, Iran.

Physiology

Stages of sleep

In mammals and birds, sleep is divided into two broad types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta, or slow-wave, sleep (SWS).

Sleep cycles through the night, with deep sleep early on and more REM (marked in red) toward morning.

Stage N3 sleep. EEG highlighted by red box.

REM sleep. EEG highlighted by red box; eye movements highlighted by red line.

Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) early in the night, while the proportion of REM sleep increases later in the night and just before natural awakening.

The stages of sleep were first described in 1937 by Alfred Lee Loomis and coworkers, who separated the different EEG features of sleep into five levels (A to E), which represented the spectrum of wakefulness to deep sleep. In 1953, REM sleep was discovered as distinct, and thus William Dement and Nathaniel Kleitman reclassified sleep into four NREM stages and REM. The staging criteria were standardized in 1968 by Allan Rechtschaffen and Anthony Kales in the "R&K sleep scoring manual." In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.

In 2004, the AASM commissioned the AASM Visual Scoring Task Force to review the R&K scoring system, which culminated in several changes, the most significant being the combination of stages 3 and 4 into Stage N3. This was published in 2007 as The AASM Manual for the Scoring of Sleep and Associated Events. Arousals and respiratory, cardiac, and movement events were also added.

Sleep stages and other characteristics of sleep are commonly assessed by polysomnography in a specialized sleep laboratory. Measurements taken include electroencephalography (EEG) of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. In humans, each sleep cycle lasts from 90 to 110 minutes on average, and each stage may have a distinct physiological function. Drugs such as sleeping pills and alcoholic beverages can suppress certain stages of sleep, leading to sleep deprivation. This can result in sleep that exhibits loss of consciousness but does not fulfill its physiological functions (i.e., one may still feel tired after otherwise sufficient sleep). REM and slow-wave sleep are both homeostatically driven; people and most animals selectively deprived of one of these stages will rebound once uninhibited sleep is allowed. This finding suggests that both these stages are essential.

NREM sleep

Main article: Non-rapid eye movement sleep

According to the 2007 AASM standards, NREM consists of three stages. There is relatively little dreaming in NREM.

Stage N1 refers to the transition of the brain from alpha waves having a frequency of 8 to 13 Hz (common in the awake state) to theta waves having a frequency of 4 to 7 Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and hypnic jerks, also known as positive myoclonus, may be associated with the onset of sleep during N1. Some people may also experience hypnagogic hallucinations during this stage, which can be troublesome to them. During N1, the subject loses some muscle tone and most conscious awareness of the external environment.

Stage N2 is characterized by sleep spindles ranging from 12 to 16 Hz and K-complexes. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45 to 55% of total sleep in adults.

Stage N3 (deep or slow-wave sleep) is characterized by delta waves ranging from 0.5 to 4 Hz (also called delta rhythms). This is the stage in which such parasomnias as night terrors, bedwetting, sleepwalking, and sleep-talking occur.

REM sleep

Main article: Rapid eye movement sleep

Rapid eye movement sleep, or REM sleep, accounts for 20–25% of total sleep time in normal human adults. The criteria for REM sleep include, obviously, rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular atonia is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.

Timing

Sleep timing is controlled by the circadian clock, homeostasis, and in humans (within certain bounds), willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one's chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.

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. Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. Sleep offset (awakening) is primarily determined by circadian rhythm. A normal person who regularly awakens at an early hour will generally not be able to sleep much later than the person's normal waking time, even if moderately sleep-deprived.

Optimal amount in humans

Adult

The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual's circadian rhythms. A person's major sleep episode is relatively inefficient and inadequate when it occurs at the "wrong" time of day. The timing is correct when the following two circadian markers occur after the middle of the sleep episode but before awakening:

• maximum concentration of the hormone melatonin, and
• minimum core body temperature.

The National Sleep Foundation in the United States maintains that seven to nine hours of sleep for adult humans is optimal and that sufficient sleep benefits alertness, memory, problem solving, and overall health, as well as reducing the risk of accidents. A widely publicized 2003 study performed at the University of Pennsylvania School of Medicine demonstrated that cognitive performance declines with six or fewer hours of sleep.

However, a University of California, San Diego psychiatry study of more than one million adults found that people who live the longest self-report sleeping for six to seven hours each night. Another study of sleep duration and mortality risk in women showed similar results. Other studies show that "sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality," though this study suggests the cause is probably other factors such as depression and socioeconomic status, which would correlate statistically. It has been suggested that the correlation between lower sleep hours and reduced morbidity only occurs with those who wake after less sleep naturally, rather than those who use an alarm.

A Kutchi woman sleeping.

Researchers from the University of Warwick and University College London have found that lack of sleep can more than double the risk of death from cardiovascular disease, but that too much sleep can also double the risk of death. Professor Francesco Cappuccio said, "Short sleep has been shown to be a risk factor for weight gain, hypertension, and Type 2 diabetes, sometimes leading to mortality; but in contrast to the short sleep-mortality association, it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated. Some candidate causes for this include depression, low socioeconomic status, and cancer-related fatigue. …In terms of prevention, our findings indicate that consistently sleeping around 7 hours per night is optimal for health, and a sustained reduction may predispose to ill health."

Furthermore, sleep difficulties are closely associated with psychiatric disorders such as depression, alcoholism, and bipolar disorder. Up to 90% of patients with depression are found to have sleep difficulties.

Hours by age

A child sleeping.

Children need more sleep per day than adults 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 or so, only slightly over two hours is spent in REM.

Age and condition	Average amount of sleep per day
Newborn	up to 18 hours
1–12 months	14–18 hours
1–3 years	12–15 hours
3–5 years	11–13 hours
5–12 years	9–11 hours
Adolescents	9–10 hours
Adults, including elderly	7–8 (+) hours
Pregnant women	8 (+) hours

Sleep debt

Main article: Sleep debt

Sleep debt is the effect of not getting enough rest and sleep; a large debt causes mental, emotional, and physical fatigue. It is unclear why a lack of sleep causes irritability; however, theories are emerging that suggest if the body produces insufficient cortisol during deep sleep, it can have negative effects on the alertness and emotions of a person during the day.

Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.

Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual's average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. It is likely that children are sleeping less than previously in Western societies.

Functions

The multiple theories proposed to explain the function of sleep reflect the as yet incomplete understanding of the subject.

It is likely that sleep evolved to fulfill some primeval function, but has taken over multiple functions over time as organisms have evolved as with the larynx which today performs multiple functions such as controlling the passage of food and air, phonation for communicating, and social purposes.

Some of the many proposed functions of sleep are as follows.

Restoration

Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al. in 2004 shows sleep deprivation hindering the healing of burns on rats.

It has also been shown that sleep deprivation affects the immune system. In a study by Zager et al. in 2007, rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats' blood tests indicated a 20% decrease in white blood cell count, a significant change in the immune system.

It has yet to be clearly proven that sleep duration affects somatic growth. One study by Jenni et al. in 2007 recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth." It has been shown that sleep—more specifically, slow-wave sleep (SWS)—does affect growth hormone levels in adult men. During eight hours' sleep, Van Cauter, Leproult, and Plat found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.

There are multiple arguments supporting the restorative function of sleep. We are rested after sleeping, and it is natural to assume that this is a basic purpose of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones (as mentioned above) are secreted preferentially during sleep. The duration of sleep among species is, in general, inversely related to animal size and directly related to basal metabolic rate. Rats with a very high basal metabolic rate sleep for up to 14 hours a day, whereas elephants and giraffes with lower BMRs sleep only 3–4 hours per day.

Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary nonsleeping animal is more likely to survive predators, while still preserving energy. Sleep, therefore, seems to serve another purpose, or other purposes, than simply conserving energy; for example, hibernating animals waking up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well-rested and are conserving energy during hibernation, but need sleep for something else. Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and eventually, septicemia and death.

Anabolic/catabolic

Non-REM sleep may be an anabolic state marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular, and skeletal systems (with some exceptions). Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and reproduces.

Ontogenesis

According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al., 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004).

REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses, yet without any motor consequences that may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.

However, this does not explain why older adults still need REM sleep. Aquatic mammal infants do not have REM sleep in infancy; REM sleep in those animals increases as they age.

Memory processing

Scientists have shown numerous ways in which sleep is related to memory. In a study conducted by Turner, Drummond, Salamat, and Brown, working memory was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic memory. Turner et al. allowed 18 women and 22 men to sleep only 26 minutes per night over a four-day period. Subjects were given initial cognitive tests while well-rested, and then were tested again twice a day during the four days of sleep deprivation. On the final test, the average working memory span of the sleep-deprived group had dropped by 38% in comparison to the control group.

Memory also seems to be affected differently by certain stages of sleep such as REM and slow-wave sleep (SWS). In one study cited in Born, Rasch, and Gais, multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and were then tested on it, both on early and late nights, with the order of nights balanced across participants. When the subjects' brains were scanned during sleep, hypnograms revealed that SWS was the dominant sleep stage during the early night, representing around 23% on average for sleep stage activity. The early-night test group performed 16% better on the declarative memory test than the control group. During late-night sleep, REM became the most active sleep stage at about 24%, and the late-night test group performed 25% better on the procedural memory test than the control group. This indicates that procedural memory benefits from late, REM-rich sleep, whereas declarative memory benefits from early, SWS-rich sleep.

Another study conducted by Datta indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed wherein a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a five-second delay, an electrical shock would be applied. Once the shock commenced, the rat could move to the other end of the box, ending the shock immediately. The rat could also use the five-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded five seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial, but the rats were shocked regardless of their reaction. After each of the training sessions, the rat would be placed in a recording cage for six hours of polygraphic recordings. This process was repeated for three consecutive days. This study found that during the posttrial sleep recording session, rats spent 25.47% more time in REM sleep after learning trials than after control trials. These trials support the results of the Born et al. study, indicating an obvious correlation between REM sleep and procedural knowledge.

Another interesting observation of the Datta study is that the learning group spent 180% more time in SWS than did the control group during the post-trial sleep-recording session. This phenomenon is supported by a study performed by Kudrimoti, Barnes, and McNaughton. This study shows that after spatial exploration activity, patterns of hippocampal place cells are reactivated during SWS following the experiment. In a study by Kudrimoti et al., seven rats were run through a linear track using rewards on either end. The rats would then be placed in the track for 30 minutes to allow them to adjust (PRE), then they ran the track with reward-based training for 30 minutes (RUN), and then they were allowed to rest for 30 minutes. During each of these three periods, EEG data were collected for information on the rats' sleep stages. Kudrimoti et al. computed the mean firing rates of hippocampal place cells during prebehavior SWS (PRE) and three ten-minute intervals in postbehavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that ten minutes after the trial RUN session, there was a 12% increase in the mean firing rate of hippocampal place cells from the PRE level; however, after 20 minutes, the mean firing rate returned rapidly toward the PRE level. The elevated firing of hippocampal place cells during SWS after spatial exploration could explain why there were elevated levels of SWS sleep in Datta's study, as it also dealt with a form of spatial exploration.

The different studies all suggest that there is a correlation between sleep and the many complex functions of memory. Harvard sleep researchers Saper and Stickgold point out that an essential part of memory and learning consists of nerve cell dendrites' sending information to the cell body to be organized into new neuronal connections. This process demands that no external information is presented to these dendrites, and they suggest that this may be why it is during sleep that we solidify memories and organize knowledge.

Further information: [[:Sleep and learning, Sleep and creativity]]

Preservation

The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the person during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk. Organisms do not require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way, where potentially they could be prey to other, stronger organisms. They sleep at times that maximize their safety, given their physical capacities and their habitats. (Allison & Cicchetti, 1976; Webb, 1982).

However, this theory fails to explain why the brain disengages from the external environment during normal sleep. Another argument against the theory is that sleep is not simply a passive consequence of removing the animal from the environment, but is a "drive"; animals alter their behaviors in order to obtain sleep. Therefore, circadian regulation is more than sufficient to explain periods of activity and quiescence that are adaptive to an organism, but the more peculiar specializations of sleep probably serve different and unknown functions.

Moreover, the preservation theory does not explain why carnivores like lions, which are on top of the food chain, sleep the most. By the preservation logic, these top carnivores should not need any sleep at all. Preservation also does not explain why aquatic mammals sleep while moving. Lethargy during these vulnerable hours would do the same and would be more advantageous, because the animal will be quiescent but still be able to respond to environmental challenges like predators, etc. Sleep rebound that occurs after a sleepless night will be maladaptive, but still occurs for a reason. For example, a zebra falling asleep the day after it spent the sleeping time running from a lion is more, not less, vulnerable to predation.

Dreaming

Main article: Dream

Dreaming is the perception of sensory images and sounds during sleep, in a sequence which the sleeper/dreamer usually perceives more as an apparent participant than an observer. Dreaming is stimulated by the pons and mostly occurs during the REM phase of sleep.

People have proposed many hypotheses about the functions of dreaming. Sigmund Freud postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the subconscious, and he used dream interpretation in the form of psychoanalysis to uncover these desires. Scientists have become skeptical about the Freudian interpretation, and place more emphasis on dreaming as a requirement for organization and consolidation of recent memory and experience . However, Freud's work concerns the psychological role of dreams, which clearly do not exclude the physiological role of consolidation of recent memory and experience. See Freud: The Interpretation of Dreams.

Rosalind Cartwright stated, "One function of dreams may be to restore our sense of competence… it is also probable that in many times of stress, dreams have more work to do in resolving our problems and are thus more salient and memorable."

John Allan Hobson and Robert McCarley's activation synthesis theory proposes that dreams are caused by the random firing of neurons in the cerebral cortex during the REM period. According to this theory, the forebrain then creates a story in an attempt to reconcile and make sense of the nonsensical sensory information presented to it; hence, the odd nature of many dreams.

Effect of food and drink on sleep

Depressants

• Alcohol

Often, people start drinking alcohol in order to get to sleep (alcohol is initially a sedative and will make you get to sleep faster). However, being addicted to alcohol can lead to disrupted sleep, because alcohol has a rebound effect later in the night. As a result, there is strong evidence linking alcoholism and insomnia.

• Barbiturates

Barbiturates cause drowsiness and have actions similar to ethanol (drinking alcohol).

• Melatonin

Melatonin is a naturally occurring hormone that regulates sleepiness. It is made in the brain, where tryptophan is converted into serotonin and then into melatonin, which is released at night by the pineal gland to induce and maintain sleep. Melatonin supplementation may be used as a sleep aid, both as a hypnotic and as a chronobiotic (see phase response curve, PRC).

• Siesta and the "post-lunch dip"

Many people have a temporary drop in alertness in the early afternoon, commonly known as the "post-lunch dip." While a large meal can make a person feel sleepy, the post-lunch dip is mostly an effect of the biological clock. People naturally feel most sleepy (have the greatest "drive for sleep") at two times of the day about 12 hours apart—for example, at 2:00 a.m. and 2:00 p.m. At those two times, the body clock "kicks in." At about 2 p.m. (14:00), it overrides the homeostatic buildup of sleep debt, allowing several more hours of wakefulness. At about 2 a.m. (02:00), with the daily sleep debt paid off, it "kicks in" again to ensure a few more hours of sleep.

• Tryptophan

The amino acid tryptophan is a building block of proteins. It has been claimed to contribute to sleepiness, since it is a precursor of the neurotransmitter serotonin, involved in sleep regulation. However, no solid data have ever linked modest dietary changes in tryptophan to changes in sleep.

Stimulants

• Amphetamines

Amphetamines (amphetamine, dextroamphetamine, methamphetamine, etc.) are often used to treat narcolepsy and ADHD disorders and are used recreationally, in which case they may be referred to as "speed." Their most common effects are decreased hunger, anxiety, insomnia, stimulation, and increased alertness. Adderall is a mixture of amphetamine salts used to treat ADHD.

• Caffeine

Caffeine is a stimulant that works by slowing the action of the hormones in the brain that cause sleepiness, particularly by acting as a antagonist at adenosine receptors. Effective dosage is individual, in part dependent on prior usage. It can cause a rapid reduction in alertness as it wears off.

• Cocaine and crack cocaine

Studies on cocaine have shown its effects to be mediated through the circadian rhythm system. This may be related to the onset of hypersomnia (oversleeping) in regard to "Cocaine-Induced Sleep Disorder."

• Energy Drinks

The stimulating effects of energy drinks come from stimulants such as caffeine, sugars, and essential amino acids, and they will eventually create a rapid reduction in alertness similar to that of caffeine.

• MDMA, including similar drugs like MDA, MMDA, or bk-MDMA

The class of drugs called empathogen-entactogens keep users awake with intense euphoria. Commonly known as "ecstasy."

• Methylphenidate

Commonly known by the brand names Ritalin and Concerta, methylphenidate is similar in action to amphetamines and cocaine.

Causes of difficulty in sleeping

There are a great many possible reasons for sleeping poorly. Following sleep hygienic principles may solve problems of physical or emotional discomfort. When pain, illness, drugs, or stress are the culprit, the cause must be treated. Sleep disorders—including the sleep apneas, narcolepsy, primary insomnia, periodic limb movement disorder (PLMD), restless leg syndrome (RLS), and the circadian rhythm sleep disorders—are treatable.

Elderly people may to some degree lose the ability to consolidate sleep. They need the same amount per day as they've always needed, but may need to take some of their sleep as daytime naps.

Anthropology of sleep

Recent research suggests that sleep patterns vary significantly across cultures. The most striking differences are between societies that have plentiful sources of artificial light and ones that do not. The primary difference appears to be that prelight cultures have more broken-up sleep patterns. For example, people might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours. The boundaries between sleeping and waking are blurred in these societies. Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterised primarily by deep sleep and the second by REM sleep. This segmented sleep has led to expressions such as "first sleep," "watch," and "second sleep," which appear in literature from preindustrial societies all over the world.

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many nomadic or hunter-gatherer societies, people will sleep on and off throughout the day or night depending on what is happening.

Plentiful artificial light has been available in the industrialised West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced. In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always true.

In some societies, people generally sleep with at least another person (often many) or with animals. In others, people rarely sleep with anyone but a most intimate relation, such as a spouse. In almost all societies, sleeping partners are strongly regulated by social standards. For example, people might only sleep with their immediate family, extended family, spouses, their children, children of a certain age, children of specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity.

People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or beds. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, and the incidence of pests.

It is recommended to adopt a sleeping position that helps maintain the curves in one's back. Sleeping on one's stomach may cause back strain. In a survey of over 1000 adults, a British researcher found that sleep posture can reveal personality traits.

Sleep in nonhumans

Sleeping Japanese Macaques.

Main article: Sleep (non-human)

Horses and other herbivorous ungulates can sleep while standing, but must necessarily lie down for REM sleep (which causes muscular atony) for short periods. Giraffes, for example, only need to lie down for REM sleep for a few minutes at a time. Bats sleep while hanging upside down. Some aquatic mammals and some birds can sleep with one half of the brain while the other half is awake, so-called unihemispheric slow-wave sleep. Birds and mammals have cycles of non-REM and REM sleep (as described above) for humans, though birds’ cycles are much shorter and they do not lose muscle tone (go limp) to the extent that most mammals do.

Many animals sleep, but neurological sleep states are difficult to define in lower-order animals. In these animals, sleep is defined using behavioral characteristics such as minimal movement, postures typical for the species, and reduced responsiveness to external stimulation. Sleep is quickly reversible, as opposed to hibernation or coma, and sleep deprivation is followed by longer and/or deeper sleep. Herbivores, who require a long waking period to gather and consume their diet, typically sleep less each day than similarly sized carnivores, who might well consume several days' supply of meat in a sitting.

Many species of mammals sleep for a large proportion of each 24-hour period when they are very young. However, killer whales and some dolphins do not sleep during the first month of life. Such differences may be explained by the ability of land-mammal newborns to be easily protected by parents while sleeping, while marine animals must, even while very young, be more continuously vigilant for predators.

William C. Dement, in his book The Promise of Sleep, states that the dolphin, originally a land mammal that has returned to the sea, maintains a number of terrestrial traits including bearing live offspring and, unlike fish, breathing air. When terrestrial mammals breathe, they do so through an involuntary process similar to the one that causes our hearts to beat continuously. In dolphins, the breathing process is under voluntary control throughout the day—a process that seemingly would preclude sleep. In order to accomplish what seems to be an impossible task, dolphins allow one half of their brain to go to sleep while the other half remains awake. This is accomplished in two-hour cycles, where one half of the brain is awake while the other half sleeps until the dolphin's daily sleep need has been fulfilled.

See also

Common sleeping positions, practices, and rituals

• Co-sleeping
• Hypnosis
• Meditation
• Neutral spine
• Sleep hygiene
• Yoga Nidra
• Siesta

Other

• Alarm clock
• Dream world (plot device)
• Microsleep
• Morvan's syndrome
• Sudden infant death syndrome

Further reading

• Bar-Yam, Yaneer (2003). "Chapter 3". Dynamics of Complex Systems. {{cite book}}: |format= requires |url= (help); External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
• Foldvary-Schaefer N, Grigg-Damberger M (2006). "Sleep and epilepsy: what we know, don't know, and need to know". J Clin Neurophysiol. 23 (1): 4–20. doi:10.1097/01.wnp.0000206877.90232.cb. PMID 16514348. {{cite journal}}: Unknown parameter |month= ignored (help)
• Gilmartin G, Thomas R (2004). "Mechanisms of arousal from sleep and their consequences". Curr Opin Pulm Med. 10 (6): 468–74. doi:10.1097/01.mcp.0000143690.94442.b3. PMID 15510052. {{cite journal}}: Unknown parameter |month= ignored (help) [Review]
• Gottlieb D, Punjabi N, Newman A, Resnick H, Redline S, Baldwin C, Nieto F (2005). "Association of sleep time with diabetes mellitus and impaired glucose tolerance". Arch Intern Med. 165 (8): 863–7. doi:10.1001/archinte.165.8.863. PMID 15851636. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
• Legramante J, Galante A (2005). "Sleep and hypertension: a challenge for the autonomic regulation of the cardiovascular system". Circulation. 112 (6): 786–8. doi:10.1161/CIRCULATIONAHA.105.555714. PMID 16087808. {{cite journal}}: Unknown parameter |month= ignored (help) [Editorial]
• Feinberg I. Changes in sleep cycle patterns with age J Psychiatr Res. 1974;10:283–306. [review]
• Dement, William C., M.D., Ph.D. The Promise of Sleep. Delacorte Press, Random House Inc., New York, 1999.
• Tamar Shochat and Sonia Ancoli - Specific Clinical Patterns in Aging - Sleep and Sleep Disorders [website]
• Zepelin H. Normal age related changes in sleep. In: Chase M, Weitzman ED, eds. Sleep Disorders: Basic and Clinical Research. New York: SP Medical; 1983:431–434.
• Morrissey M, Duntley S, Anch A, Nonneman R (2004). "Active sleep and its role in the prevention of apoptosis in the developing brain". Med Hypotheses. 62 (6): 876–9. doi:10.1016/j.mehy.2004.01.014. PMID 15142640.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Marks G, Shaffery J, Oksenberg A, Speciale S, Roffwarg H (1995). "A functional role for REM sleep in brain maturation". Behav Brain Res. 69 (1–2): 1–11. doi:10.1016/0166-4328(95)00018-O. PMID 7546299. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
• Mirmiran M, Scholtens J, van de Poll N, Uylings H, van der Gugten J, Boer G (1983). "Effects of experimental suppression of active (REM) sleep during early development upon adult brain and behavior in the rat". Brain Res. 283 (2–3): 277–86. PMID 6850353. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
• Zhang, J. (2004). "[Memory process and the function of sleep]" (PDF). Journal of Theoretics. 6 (6). {{cite journal}}: Unknown parameter |month= ignored (help)

External links

• American Academy of Sleep Medicine
• American Sleep Association
• Healthy Sleep from the Division of Sleep Medicine at Harvard Medical School and WGBH Educational Foundation
• Is Sleep Essential? from the Public Library of Science Biology
• National Center on Sleep Disorders Research
• National Sleep Foundation
• Sleep Research Society
• Sleep Review