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In humans, other mammals, and a substantial majority of other animals which have been studied by humans — such as fish, birds, ants, and fruit-flies — regular sleep is necessary for survival. While sleep is essential for survival, there currently does not exist credible scientific evidence behind the purpose of sleep.
Physiology (Stages of Sleep)
In mammals, the measurement of eye movement during sleep is used to divide sleep into two broad types:
Each type has a distinct set of associated physiological, neurological and psychological features.
Sleep proceeds in cycles of REM and NREM phases. In humans, this cycle is approximately 90 to 120 minutes. Each phase may have a distinct physiological function. Drugs such as alcohol and sleeping pills 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.
Rechtschaffen and Kales originally outlined the criteria for staging sleep in 1968. The American Academy of Sleep Medicine (AASM) updated the staging rules in 2007.
Non-REM (NREM) Sleep
NREM accounts for 75–80% of total sleep time in normal human adults. In NREM sleep, the body is active and the brain is inactive, and there is relatively little dreaming. Non-REM encompasses four stages; stages 1 and 2 are considered 'light sleep', and 3 and 4 'deep sleep'. They are differentiated solely using EEG, unlike REM sleep which is characterized by rapid eye movements and relative absence of muscle tone. There are often limb movements, and parasomnia sleep walking occurs in non-REM sleep. 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 human is selectively deprived of one of these, it rebounds once the person is allowed to sleep. This suggests that both are essential in the sleep process and its many functions.
Despite decades of research, knowledge of sleep's function is minimal. Several theories and axioms are listed herein.
Sleep affects the body in several ways. Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al.  shows sleep deprivation hindering the healing of burns on rats. The subjects were 50 male rats, and they all received full-skin-thickness burns. They were then divided in 5 groups of 10, but only two groups are important to the claim; they are the control and sleep deprived group. The experimental group of rats was then sleep deprived for 72 hours by switching wood shavings in the cages with water. After the sleep deprivation period, rats were evaluated to determine the extent of wound healing that occurred. A scale with scores from 0 to 3, no cells to countless cells, was used to evaluate the amount of polymorphonuclear leucocytes (PNLs, a type of white blood cell), immunoglobulin G (IgGs, a type of antibody), fibroblasts (a cell that maintains tissue and play an important role in wound healing), and proliferation of capillaries in samples . When compared to the control, there was a score decrease of 78% in fibroblasts, an 89% score decrease in capillaries, a 92% score increase in PNLs levels, and a 34% score increase in IgG levels. These changes in wound healing from sleep deprivation indicate that sleep is related to healing. It’s also been shown that sleep deprivation affects the immune system and metabolism. In a study by Zager, Andersen, Ruiz, Antunes, and Tufik  10 ninety day old 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, which is a significant change in the immune system. Another study, this one conducted by Bonnet and Arand , indicates that sleep affects metabolism. Eighteen human subjects were chosen for this study and paired by sex, age, and weight, 4 females and 14 males. One of the two in each pair had a diagnosed form of insomnia called sleep state misperception, and the other member was a normal sleeper. All subjects stayed at the lab for two nights and had standard clinical polysomnograms performed on them. Metabolic rate was measured on the second night alone. The researchers defined metabolic rate in their study as a measure of overall oxygen use (VO2 measured in mL min-1), as they felt it was the best indicator of physiological activity. It was found that the insomniacs slept only 6.8 hours a night to the normal sleepers' 7.5 hours. The overall metabolism mean value for the insomniacs was 286 mL min-1 where as the normal sleepers mean value was 304 mL min-1, a significant difference.
It has yet to be clearly proven that sleep affects somatic growth. One study by Jenni, Molinari, Caflisch, and Largo  recorded growth, height and weight, as it correlates to sleep in 305 children. These children were volunteered by their parents and information was collected from age 1 to 10. The subject’s guardians were interviewed at regular intervals to obtain information on their child’s sleep duration. Jenni et al.  found the mean sleep duration of the subjects decreased with age by about 8% per year while mean height increased by 4% and mean weight increased by 14% per year. It has been shown that sleep, more specifically slow-wave sleep (SWS), does affect growth hormone levels. A study by Van Cauter, Leproult, and Plat  used 149 healthy men as subjects ages 16 to 83. Blood samples were taken for growth hormone measurement every 15 to 30 minutes for 24 hours. At night subjects were allowed to sleep for 8 hours and all-night polygraphic sleep recordings were obtained. The results showed that subjects with high SWS (average SWS percentage of 24%) also had high growth hormone secretion, an average of 275µg. Subjects with low SWS (average SWS percentage of 9%) also had low growth hormone secretion, an average of 150µg.
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 (but see above). Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and procreates.
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).
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. This demonstrates that there is clearly a connection between sleep and memory.
Memory also seems to be affected differently by certain stages of sleep such as rapid eye movement sleep (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 subject’s 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 where as 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 Kudrimonti, 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 Kudrimonti 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 was collected for information on the rats’ sleep stages. Kudrimonti et al.  computed the mean firing rates of hippocampal place cells during pre-behavior SWS (PRE) and three 10 min intervals in post-behavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that 10 min 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.
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. A seemingly more advantageous adaptation for animals would be to seclude themselves but remain quietly awake to avoid predation. In fact, animals who are preyed upon usually disengage from the external environment to a lesser degree. 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.
These theories are not mutually exclusive; each may contain truths that may be validated in the future. Recent studies show that sleep is phylogenetically ancient (Shaw et al Science 2000, Hendricks et al Neuron 2000). Thus, to understand the function of sleep, we must study simple animals that predated the arthropoda and chordata phyla, as well as the roles of proteins and enzymes in basic metabolism. Some sleep features are unique to mammals (e.g. REM sleep and thermoregulation) and thus probably did not occur in sleeplike states of primordial metazoa.
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:
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.
A University of California, San Diego psychiatry study found that people who live the longest sleep for six to seven hours each night. However, this study cannot be used to determine optimal sleep habits, only correlation — and empirically observed correlation is a necessary but not sufficient condition for causality. For example, such correlation can be explained from the fact that older people tend to sleep less, or perhaps a genetic ability to generate cells faster provides advantages in both sleep necessity and longevity.
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.”
Children need a greater amount of sleep than adults to function correctly (up to 18 hours for newborn babies, with a declining rate as the child ages).
Longest period without sleep
Depending on how one defines sleep, there are several people who can claim the record for having gone the longest without sleep.
Causes of difficulty sleeping
Many people have trouble sleeping, which may stem from a number of issues, including:
A study by researchers at the University of Pennsylvania has confirmed that the more one works, the less one sleeps - and that work is the single biggest factor troubling sleep.
Dreaming is the perception of sensory images 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
James 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.
A wet dream is the ejaculation of semen during sleep. This is most often experienced by pubescent boys during REM sleep, but may occur at any time after puberty.
Anthropology of sleep
Recent research suggests that sleep patterns vary significantly across human cultures. The most striking differences are between societies that have plentiful artificial light and ones that do not. Cultures without artificial light have more broken-up sleep patterns. This is called segmented sleep, which has led to expressions such as "first sleep," "watch," and "second sleep" which appear in literature from all over the world.
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.
Some sleep deprivation-oriented sleep patterns have been experimented with recently, such as that of the Uberman's sleep schedule, which involve sleeping in regular patterns of 20 minute sleep and 4 hours awake, leading to greatly increased wake time. Such patterns purportedly lead to the body's ability to jump instantly into the most necessary sleep stages.
Since plentiful artificial light became available in some cultures in the mid-19th century, sleep patterns have changed significantly in these cultures. These people sleep in a concentrated burst at night, and sleep later in the morning.
Cattle, horses, and sheep can sleep while standing or while lying down; however, they cannot experience REM sleep while standing. If deprived of REM sleep for a long time, the animal may involuntarily collapse in order to reach REM sleep, a condition not to be confused with narcolepsy. Whales and dolphins are also different from humans: they always have to be conscious, as they are conscious breathers, so only one half of their brain sleeps at a time. Sleep becomes difficult to define in lower order animals, such as the bullfrog. Its resting state is too similar to its active state to be considered by many to satisfy the criteria for sleep, but brain activity in the resting state is similar to other amphibians that do meet the criteria when they sleep.
Patterns and disruptions
Practices and rituals
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sleep". A list of authors is available in Wikipedia.|