Sleep is a natural state of bodily rest observed throughout the animal kingdom. 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 fish, birds, ants, and fruit-flies — regular sleep is essential for survival. However, its purposes are only partly clear and are the subject of intense research.
In mammals and birds the measurement of eye movement during sleep is used to divide sleep into the two broad types of Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM) sleep. Each type has a distinct set of associated physiological, neurological and psychological features.
Sleep proceeds in cycles of REM and the four stages of NREM, the order normally being:
In humans this cycle is on average 90 to 110 minutes, with a greater amount of stages 3 and 4 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 (see Sleep deprivation). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.
Allan Rechtschaffen and Anthony Kales originally outlined the criteria for identifying the stages of sleep in 1968. The American Academy of Sleep Medicine (AASM) updated the staging rules in 2007.
Criteria for REM sleep include not only rapid eye movements but also a rapid low voltage EEG. In mammals, at least, low muscle tone is also seen. Most memorable dreaming occurs in this stage. NREM accounts for 75–80% of total sleep time in normal human adults. In NREM sleep, there is relatively little dreaming. Non-REM encompasses four stages; stages 1 and 2 are considered 'light sleep', and 3 and 4 'deep sleep' or slow-wave sleep, SWS. They are differentiated solely using EEG, unlike REM sleep which is characterized by rapid eye movements and relative absence of muscle tone. In non-REM sleep there are often limb movements, and parasomnias such as sleepwalking may occur. A cyclical alternating pattern may sometimes be observed during a stage.
NREM consists of four stages according to the 2007 AASM standards:
Both REM sleep and NREM sleep stages 3 and 4 are homeostatically driven; that is, if a person or animal is selectively deprived of one of these, it rebounds once uninhibited sleep again is allowed. This suggests that both are essential to the functions of the sleep process.
Sleep timing is controlled by the circadian clock, by homeostasis and, in humans, 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, is also important and 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.
The National Sleep Foundation in the United States maintains that eight 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.
Causal links are currently speculative: the available data may only reflect comorbid depression, socioeconomic status, or even alcohol use, for example. These studies cannot be used to determine optimal sleep habits, only correlation — and empirically observed correlation is a necessary but not sufficient condition for causality. A need for nine or ten hours of sleep a day, or only five to six, may or may not have the same cause as the shortened life span. In other words, long or short sleep duration itself has not been shown to be a cause of early death.
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.
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 half of its sleep time in REM-sleep. By the age of five or so, only a bit over two hours are spent in REM.
|Age||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|
|Adults, including elderly||7-8 (+) hours|
|Pregnant women||8 (+) hours|
Sleep debt is the effect of not getting quite 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 stage 3 and 4 sleep it can have negative effects on our alertness and emotions during the day.
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.
It is likely that sleep evolved to fulfill some primeval function, but has taken over multiple functions over time as organisms have evolved. An analogy would be that to the larynx, which performs multiple functions, such as controlling the passage of food and air, phonation for communicating and social purposes, etc. These are all functions of the larynx, but just one of them likely represents the original function. Some of the many proposed functions of sleep are as follows:
It has also been shown that sleep deprivation affects the immune system and metabolism. 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.
A study by Bonnet and Arand in 2003 indicates that sleep affects metabolism, is indeed a metabolic phase - anabolism. Comparing normal human sleepers and sleepers with sleep state misperception insomnia, where patients complain of poor sleep but have normal sleep by electroencephalographic (EEG) criteria, the researchers found significantly greater metabolism values for the normal sleepers.
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. Sleep among species is, in general, inversely related to the animal size and 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 non-sleeping animal is more likely to survive predators, while still preserving energy. Sleep therefore does something else other than conserving energy. Most interestingly, hibernating animals that wake 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.
REM sleep appears to be important for development of the brain. REM sleep occupies majority of time of sleep of infants, which 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 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.
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.
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
J. 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.
The amino acid tryptophan is a building block of the protein found in foods. It contributes to sleepiness. Carbohydrates make tryptophan more available to the brain, which is why carbohydrate-heavy meals containing tryptophan tend to cause drowsiness. Tryptophan is a precursor to the neurotransmitter serotonin, which is a precursor to the neurohormone melatonin (see below).
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).
5-HTP, the precursor to serotonin (5-HT) can cause drowsiness when ingested.
Many people have a temporary drop in alertness in early afternoon, commonly known as the post-lunch dip. While a large meal, rich in carbohydrates, 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.
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).
Caffeine is a stimulant that works by slowing the action of the hormones in the brain that cause sleepiness. Effective dosage is individual, in part dependent on prior usage. It can cause a rapid reduction in alertness as it wears off.
The stimulating effects of energy drinks comes from natural stimulants such as caffeine, sugars, and essential amino acids, and eventually will create a rapid reduction in alertness similar to that of caffeine.
Amphetamines (amphetamine, dextroamphetamine, methamphetamine, etc) are often used to treat narcolepsy and ADHD disorders, the most common effects are decreased appetite, stimulation and insomnia, and increased alertness.
Similar in action to the amphetamines.
Commonly known as Ritalin, similar in action to amphetamines and cocaine.
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.
Some cultures have fragmented sleep patterns in which people sleep at all times of the day, and for shorter periods at night. For example, many Mediterranean and Latin American cultures have a siesta, in which people sleep for a period in the afternoon. In many nomadic or hunter-gatherer societies people sleep off and on throughout the day or night depending on what is happening.
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 setting.
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.