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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 which 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 demonstrated that cognitive performance in humans declines with fewer than eight hours of sleep.
Discussion
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Quotations
A good laugh and a long sleep are the best cures in the doctor's book.
A ruffled mind makes a restless pillow.
Charlotte Brontë
All men whilst they are awake are in one common world: but each of them, when he is asleep, is in a world of his own.
Consciousness: that annoying time between naps.
Death, so called, is a thing which makes men weep: And yet a third of Life is passed in sleep.
Even where sleep is concerned, too much is a bad thing.'.
Encyclopedia
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 which 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 demonstrated 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.
Physiology
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.
Sleep proceeds in cycles of REM and the three stages of NREM, the order normally being:
- stages N1 -> N2 -> N3 -> N2 -> REM.
(Until recently, NREM was considered to include four stages and the cycle was N1 -> N2 -> N3 -> N4 -> N3 -> N2 -> REM; this system is still seen in reference works and diagrams.)
In humans each sleep cycle lasts on average 90 to 110 minutes, with a greater amount of stage 3 (N3) early in the night and more REM later in the night. Each phase 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.
Stages
Criteria for staging, or scoring the stages of sleep, have been changed several times. First described in 1937 by Loomis et al, the staging was changed in 1957 after the discovery of rapid eye movement (REM) sleep, standardized in 1968 by Rechtschaffen & Kales (R&K) and most recently in 2007 in The AASM Manual for the Scoring of Sleep and Associated Events published by the American Academy of Sleep Medicine (AASM). In addition to adding arousals and respiratory, cardiac, and movement events, the major change in 2007 was the elimination of Stage 4: in the R&K standard, the difference between Stages 3 and 4 (now combined as 3) was that delta waves made up less than 50% of the total wave-patterns in stage 3, while they were more than 50% in stage 4. The R&K standard included four stages of NREM sleep plus REM, sometimes referred to as "stage 5". The newer standard discontinues stage 4 sleep and leaves only stage 3 to describe deep sleep, also known as delta or slow-wave sleep.
Sleep stages and other characteristics of sleep can be measured by polysomnography in a sleep laboratory using among other tools electroencephalography (EEG) for brain waves, electrooculography (EOG) for eye movements and electromyography (EMG) for activity of skeletal muscles.
Criteria for REM sleep include not only rapid eye movements but also rapid low voltage EEG, commonly called brain waves. In mammals at least, low muscle tone is also seen, often called paralysis. Most memorable dreaming occurs in this stage. REM sleep accounts for 20–25% of total sleep time in normal human adults; NREM (non-REM) accounting for the rest. In NREM sleep, there is relatively little dreaming. Non-REM encompasses three stages; stage 1 (N1), stage 2 (N2), and stage 3 (N3); N3 being referred to as 'deep sleep' or slow-wave sleep (SWS). Sleep stages are differentiated by brain waves, eyes movements, and skeletal muscle activity.
NREM consists of three stages according to the 2007 AASM standards:
- During Stage N1 the brain transitions from alpha waves (having a frequency of 8 to 13 Hz, common to people who are awake) to theta waves (with a frequency of 4 to 7 Hz). This stage is sometimes referred to as somnolence, or "drowsy sleep". Associated with the onset of sleep during N1 may be sudden twitches and hypnic jerks also known as positive myoclonus. 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" (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 (SWS), is characterized by delta waves, (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 and SWS sleep (previously stages 3 and 4, now combined as stage 3) are both homeostatically driven; that is, people and most animals selectively deprived of one of these stages will rebound once uninhibited sleep is allowed. This finding suggests that both types of sleep are essential.
Timing
Sleep timing is controlled by the circadian clock, by homeostasis and in humans, within certain bounds, by willed behavior. The circadian clock, an inner time-keeping, temperature-fluctuating, enzyme-controlling device, works in tandem with adenosine, a neurotransmitter which inhibits many of the bodily processes that are 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 which 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 and 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 fewer than eight 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 socio-economic 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.
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
Children need a greater amount of 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 are spent in REM.
| Age and condition | Average amount of sleep per day |
|---|
| 1-6 months | 16-16.5 hours | | 6-18 months | 14-16 hours | | 18 months-4 years | 12-14 hours | | 4-7 years | 11 hours | | 7-10 years | 10-10.5 hours | | 10-13 years | 10 hours | | 13-16 years | 9-10 hours | | 16 years and above | 8-9 hours |
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 the 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 where as 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 non-sleeping 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 which 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 4-day period. Subjects were given initial cognitive tests while well rested and then tested again twice a day during the 4 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 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 where 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 5 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 5-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded 5 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 6 hours of polygraphic recordings. This process was repeated for 3 consecutive days. This study found that during the post-trial 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 pre-behavior SWS (PRE) and three 10-minute intervals in post-behavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that 10 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.
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 does not explain why aquatic mammals sleep while moving. Lethargy during these vulnerable hours would do the same, and will 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 and not less vulnerable to predation.
Dreaming
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. See Freud: The Interpretation of Dreams
Rosalind Cartwright stated that "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's 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 the 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
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 when taken cause drowsiness and have actions similar to ethanol (drinking alcohol).
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 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 AM and 2:00 PM. At those two times, the body clock "kicks in". At about 2 p.m. (14:00), it overrides the homeostatic build-up 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.
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 has ever linked modest dietary changes in tryptophan to changes in sleep.
Stimulants
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 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.
Studies on cocaine have shown its effects to be mediated through the circadian rhythm system. This may be related to the onset of hypersomnia (over sleeping) in regard to "Cocaine-Induced Sleep Disorder"
The stimulating effects of energy drinks comes from stimulants such as caffeine, sugars, and essential amino acids, and eventually will create a rapid reduction in alertness similar to that of caffeine.
The class of drugs called empathogen-entactogens keep users awake with intense euphoria. They are commonly known as ecstasy.
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 pre-light 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 wakefullness, perhaps lasting several hours. The boundaries between sleeping and waking are blurred in these societies. Some observers believe that 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 pre-industrial 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. For example, many Mediterranean societies have a siesta, in which people sleep for a period in the afternoon. In many nomadic or hunter-gatherer societies people will sleep off and on 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-nineteenth 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 one other 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, with their children, with 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 head rests, 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.
Sleep in non-humans
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 same 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
Other
Further reading
[Review]
[Editorial]
- Feinberg I. Changes in sleep cycle patterns with age [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 - - 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.
External links
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