Sleep


 * This article is about the state of rest known as sleep. For the band, see Sleep (band).

Sleep is the regular state of natural rest observed in all mammals, birds and fish. Sleep is not actually "unconsciousness," but rather, it is a natural state of rest characterized by a reduction in voluntary body movement and decreased awareness of the surroundings. Therefore, since consciousness is literally the awareness of the surroundings, being asleep is just an altered state of consciousness, as opposed to being unconscious. It is heavily influenced by circadian rhythms, and by hormonal and environmental factors as well. Sleep appears to perform a restorative function for the brain and body, as evidenced by the myriad symptoms of metabolic dysfunction that result when animals are deprived of sleep (Gottlieb et al., 2005).

The function of sleep in health and in disease is being increasingly studied in specialized sleep laboratories throughout the world. Not only insomnia, but more recently elucidated sleep disorders such as sleep apnea and narcolepsy are evaluated in such facilities. The increasing prevalence of sleep disorders is likely to be a function both of more sophisticated diagnostic tests and the disruption of the normal day-night cycle in modern societies.

Methodology
Before advances in the fields of neurology, neuroscience, electronics and genetics were made, scientists studied the behavioral characteristics of sleep, such as its pattern, depth, and varying frequency. In more recent times, the electrical impulses generated by the brain are recorded using a device called an electroencephalograph (EEG), and individual genes relating to sleep-related brain function, such as the circadian rhythm, have been isolated. Molecular biology, medical science and epidemiology all play an important role in modern studies of sleep.

Sleep is often defined using specific criteria relating to EEG data. All mammals and birds fulfill the criteria for sleep based on EEG recordings. In animals where EEG data is not readily available, or their small size precludes recording an EEG, behavioral and gene specific data are utilized for sleep studies.

Sleep regulation
The cycle of sleep and wakefulness is regulated by the brain stem, external stimuli, and various hormones produced by the hypothalamus. Some neurohormones and neurotransmitters are highly correlated with sleep and wake states. For example, melatonin levels are highest during the night, and this hormone appears to promote sleep. Adenosine, a nucleoside involved in generating energy for biochemical processes, gradually accumulates in the human brain during wakefulness but decreases during sleep. Researchers believe that its accumulation during the day encourages sleep. The stimulant properties of caffeine are attributed to its negating the effects of adenosine.

The suprachiasmatic nucleus (SCN) of the hypothalamus plays an important role in the regulation of circadian rhythms. The SCN is influenced by external light and also generates its own rhythm in isolation. In the presence of light it sends messages to the pineal gland that instruct it to cease secreting melatonin.

Thus, three processes, each influenced by hormonal, neurological, and environmental factors, underlie sleep regulation:


 * A homeostatic process determined by prior sleep and wakefulness, determining "sleep need".
 * A circadian process determining periods of high and low sleep propensity, and high and low rapid eye movement (REM) sleep propensity.
 * An ultradian process

The interrelationships and relative importance of each process and system remain uncertain.

Stages of sleep


Studies of human sleep have established five well-defined stages, according to electroencephalographic (EEG) recordings and polysomnography:


 * Non REM sleep (NREM), which accounts for 75-80% of total sleep time:
 * Stage 1, with near-disappearance of the alpha waves seen in awake states, and appearance for the first time of theta waves. The stage is sometimes referred to as somnolence, or "drowsy sleep". It appears at sleep onset (as it is mostly a transition state into Stage 2), and can be associated with so-called hypnagogic hallucinations. In this period, the subject loses some muscle tone, and conscious awareness of the external environment: Stage 1 can be thought of as a gateway state between wake and sleep.
 * Stage 2, with "sleep spindles" (12–16 Hz) and "K-complexes". The EMG lowers, and conscious awareness of the external environment all but disappears. This occupies 45-55% of total sleep.
 * Stage 3, with delta waves, also called delta rhythms (1–2 Hz) is considered part of SWS and functions primarily as a transition into stage four. Overall it occupies 3-8% of total sleep time.
 * Stage 4 is true delta sleep. It predominates the first third of the night and accounts for 10-15% of total sleep time. This is often described as the deepest stage of sleep; it is exceedingly difficult to wake a subject in this state. This is the stage in which night terrors and sleepwalking occur.


 * Stage 5, or Rapid eye movement (REM) sleep, associated with dreaming, especially bizarre, visual, and seemingly random dreams. REM sleep is predominant in the final third of a sleep period, its timing linked to circadian rhythm and body temperature. The EEG in this period is aroused and looks similar to stage 1, and sometimes includes beta waves.

Sleep proceeds in cycles of NREM and REM phases. In humans, the cycle of REM and NREM is approximately 90 minutes. Each stage may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep (see sleep deprivation below). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.

Each sleep stage is not necessarily uniform. Within a given stage, a cyclical alternating pattern may be observed.

Theories regarding the function of sleep
Restorative theories of sleep describe sleep as a dynamic time of healing and growth for organisms. For example, during stages 3 and 4, or slow wave sleep, growth hormone levels increase, and changes in immune function occur. The myriad illnesses associated with sleep deprivation testify to its restorative function.

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). Given sleep's heterogeneous nature, however, no single theory predominates, as it is difficult to describe one single "function" of sleep.

One process known to be highly dependent on sleep is memory. REM sleep appears to help with the consolidation of spatial and procedural memory, while Slow-wave sleep helps with the consolidation of declarative memories. When experimental subjects are given academic material to learn, especially if it involves organized, systematic thought, their retention is markedly increased after a night's sleep. Mere rote memorization is retained similarly well with or without an intervening period of sleep.

Non-REM sleep is an anabolic state marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular, and skeletal systems. Sleep also restores neurons and increases production of brain proteins and certain hormones. Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and procreates. Asking the question "Why do we awaken?" instead of "Why do we sleep?" yields a different perspective toward understanding how sleep and its stages contribute to a healthy organism.

Dreaming
The demonstrably necessary phenomenon of dreaming would suffice to prove the importance of sleep to humans, and perhaps to other animals as well. Dreaming involves an involuntary conjuring up of sometimes magical images in a story-like sequence in which the sleeper/dreamer is usually more a participant than an observer. Most scientists agree that dreaming is stimulated by the pons and occurs during the REM phase of sleep.

Many functions have been hypothesized for dreaming. Freud postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the subconsciousness, and used dream interpretation in the form of psychoanalysis he pioneered. Scientists today have become more skeptical about details of Freudian interpretation, and place more emphasis on dreaming as a requirement for organization and consolidation of recent memory and experience. Another theory is that dreaming allows an animal to play out scenarios that may help the animal avoid dangers when awake. For example, a rabbit might dream about being cornered by a lion and may play out different scenarios that might increase chances of survival should he come across a lion in reality.

Sleep deprivation
A common misperception is that everyone needs eight hours of sleep. The amount of sleep needed is different for each person. This amount needed is individually and biologically determined. Some can do with six hours of sleep, others need nine. However, as a general rule, eight hours is recommended. Sleep experts state that you cannot "store" sleep by sleeping more on the weekends in preparation for the normal work week. Just how much sleep deprivation leads to death in humans is unknown.

Another commonly held view is that the amount of sleep one requires decreases as one ages, but this is not necessarily the case. The ability to sleep, rather than the need for sleep, appears to decrease when people get older. 

Failure to sleep results in progressively severe psychological and physical distress. In 1965, California teenager Randy Gardner attempted to resist sleep in an uncontrolled "experiment". As his ordeal progressed he fell into a silent stupor, bringing into doubt whether he was actually awake in any practical sense. There are occasional stories of people who are able to function with a small or no amount of sleep, in many cases due to brain damage brought on by an accident, but these cases do not appear to hold up under controlled conditions.

A study at the University of Pittsburgh School of Medicine showed that poor sleep and sleep deprivation in older adults can lead to earlier death, but another survey of more than a million people in the 1980s found that those who slept more than seven and a half hours tended to die a little earlier. It is difficult to perform such studies as controlled experiments, since persons with various physical and psychological problems may be inclined to sleep longer as a result of these illnesses. For example, long sleepers tend to have a higher than average rate of sleep disorders, possibly artificially increasing their reported sleep time, and providing a statistical selection effect that undermines the validity of many such studies.

A 1999 University of Chicago team led by Eve Van Cauter limited a group of lean young men to four hours of sleep per night for 16 days. The subjects showed decreased levels of leptin and increased levels of cortisol. The subjects also increased their daily caloric intake by 1,000 calories. The team discovered that the subjects' insulin and blood sugar levels resembled the impaired glucose tolerance of prediabetics, an indication that they were no longer properly processing carbohydrates. Studies have also linked sleep deprivation to an increased incidence of obesity.

At Harvard Medical School, researchers have identified associations between sleep deprivation and illnesses ranging from hypertension and heart attacks to cancer. Poor sleepers generate increased levels of stress hormones and show more inflammatory changes in the walls of their small blood vessels, both of which contribute to elevated blood pressure. Because of their exposure to light at night, night-shift workers produce less melatonin, a hormone which not only promotes sleep but has been shown to have cancer-prevention benefits as well.

Despite the risks, sleeping less is attractive to some because of the additional time made available, and many people feel they have to sleep less to maintain their lifestyle. They may resort to trying polyphasic sleep, a method for minimizing the time spent asleep, while maximizing its effects. This is done by sleeping in short naps throughout the day, minimizing the time spent awake between periods of sleep and thus decreasing the workload of the brain while sleeping. People who take so-called power naps during the day are practicing a variation of polyphasic sleeping. In this regard, cultures where the siesta is customary are also taking advantage of alternative means of obtaining sufficient sleep.

Experiments with rats have been designed to measure the effects of severe sleep deprivation. In one, a pair of rats were placed on a platform and separated by a movable wall. Both were instrumented with electroencephalograms. Whenever the "subject" rat began to show signs of sleep the partition was moved, forcing both rats to move. The "control" rat, however, was allowed to sleep in between movements. After several weeks the "subject" rat became unable to regulate body temperature; even if allowed to sleep at this point, it died shortly afterward from septic shock.

Some recent studies concluded the cause of death in the rat experiments to be more closely related to REM deprivation, but also found the rats died in about a week less time. It is believed this is because, unlike non-REM sleep that repairs parts of the brain damaged by metabolism and free radicals, REM sleep repairs the repair center. It is unclear the degree to which the results of sleep deprivation in rats can be generalized to humans.

In sleep-deprived states less extreme than that suffered by Randy Gardner, humans display irritability, impaired cognitive function, and poor judgment. Experiments on sleep-deprived medical trainees, for example, have shown them less able to interpret EKGs and x-rays than their well rested peers. As late as early 21st century people thought that too little sleep could be negated by "paying back the sleep debt". However, recent studies have shown this to be false. After extensively prolonged period of awareness, average humans can sleep comfortably for as long as 14 hours in row, but any amount over that has no effect for health. Sleeping over it causes dizziness, lack of muscular control, numbness and several other symptoms often confronted with too little sleep.

That one major function of sleep is consolidation and optimization of memories (including "unlearning") is evidenced by studies showing that mental functions are impaired by sleep deprivation and that sleep deprivation can even be lethal. Adequate rest and a properly functioning immune system are closely related. Sleep deprivation compromises the immune system by altering the blood levels of specialized immune cells and important proteins called cytokines, resulting in an increased chance of infection. We may begin to understand why sleep deprivation is lethal if we understand sleep as a necessary period of anabolic activity for all animals, and the basic need for anabolic activity as a prerequisite for life itself.

Sleep disorders
Disorders of sleep are broadly classified into three groups. Dyssomnias are characterized by difficulty getting to sleep, as in primary insomnia, narcolepsy, and restless legs syndrome. Obstructive sleep apnea, a condition that is being diagnosed with increased frequency, may be classified either as a dyssomnia or as an example of a parasomnia. The latter conditions involve bothersome awakenings during sleep, and also include bruxism and sleepwalking. The third group includes sleep disorders resulting from a number of psychiatric problems, such as bipolar disorder, depression, or schizophrenia.

Many sleep disorders result from errors in synchronization of sleep with the body clock. Other sleep problems are organic and cannot be resolved with chronotherapy. One often effective solution to some kinds of insomnia involves free-running sleep. Free-running sleep entails ignoring alarm clocks and schedules in order to sleep when, and only when, tired. Free-running sleep can resolve the majority of synchronization-dependent sleep disorders, but is difficult to sustain due to the resulting loss of synchronization of sleep with the outside world (including the day-night cycle).

Animal sleep
Animals vary widely in their amounts of sleep, from 2 hours a day for giraffes to 20 hours for bats. Generally, required sleeping time decreases as body size increases. Cats are one of the few animals that do not have most of their sleep consolidated into one session, preferring instead to spread their sleep fairly evenly throughout the day.

Water mammals "sleep" with alternate hemispheres of their brains asleep and the other awake. They need to do this so they can breathe above water while sleeping. Migratory birds also seem to sleep this way.

Even fish and fruit flies appear to have a "sleeplike" state. This alternation of the sleeplike state and its absence is referred to as a "Basic Rest and Activity Cycle", or BRAC. Since the modern definition of sleep is defined using EEG criteria, and such tiny brains preclude the recording of EEG's, this may not technically be described as sleep. However, if fruit flies are repeatedly disturbed so that they can not rest, they have what is referred to as a "rest rebound". This behavior is strikingly similar to that exhibited by mammals and birds in similar conditions. As research equipment improves, the definition of sleep may soon be revised.

Many animals hibernate in a deep sleeplike state during winter to conserve body heat and energy. Estivation is a similar state in which other animals hibernate to escape the heat of summer.

Cattle, horses, and sheep are unique in that they can sleep while standing, though for cattle and sheep, REM sleep will not occur in such a position. For REM sleep to take place, the animals must lie down. Sleeping while standing is thus only partial sleep. However, birds may have periods of REM sleep while perched.

Approaches to sleeping better
Many non-pharmacological approaches exist to improve falling asleep and staying asleep. Doctors and health professionals may suggest any of the following, depending on the type of sleep disruption, the person's situation, and their specific sleep needs.


 * Decrease the light level in the sleeping environment. Studies have indicated that the brain has a separate neural pathway to the optical nerve, separate from the visual path, to detect whether it's day or night. This detection system could have a direct effect on successful sleep inducement. Moreover, other studies have shown sleep inducement is dramatically increased by reductions in light level, in the sleeping environment. Use appropriate curtains and shading to keep light out or at minimal levels.
 * Set a quiet time approximately 30 minutes before bedtime—no computer, television, video games, office work, housework, or other stressful, dutiful, or mentally stimulating activities—to slow down the metabolic rate.
 * Reading or other light mental activity at bedtime.
 * Milk contains tryptophan, which may help relax the nervous system and induce drowsiness. However, this theory is likely rooted in a folk remedy, and scientific evidence suggests that simply drinking milk may or may not help.
 * Get up to do some quiet activity or slowly walk around until feeling tired, if one does not fall asleep in bed after 20 to 30 minutes. Paradoxically, this may increase restlessness in some people.
 * Make sure one's sleeping position or posture is comfortable and provides enough support, especially for the lower back.
 * Quiet, slow-paced, simplistic music can also help sleep inducement. Audio music products are available on the market that are designed to help fulfill this function.
 * Avoid using the bed for activities other than sleep, to maintain an association between getting into bed and sleeping.
 * Avoid certain drugs (e.g., cocaine and Ritalin), which are stimulants and can adversely affect sleep. Many non-stimulant medications, including glucocorticoids, may also cause sleeping problems.
 * Avoid coffee, tea, soft drinks and beverages containing caffeine. (One exception may be green tea, which some claim calms nerves and helps to induce sleep.)
 * Decrease in body temperature can also help. Studies have shown sleep inducement is increased when body temperature is lowered.
 * Avoid a large evening meal, especially if consumed within four hours of bedtime. Bulky meals may lead to abdominal discomfort, nausea, or heartburn which disrupt sleep.
 * Avoid vigorous physical activity just before going to bed.
 * Avoid excessive stress and worrying, especially in the hours just before bedtime.
 * Controlled crying is used as a method of increasing the length of time babies or young children will sleep.

Sleep aiding
The pharmacological approach to inducing sleep involves the use of depressant drugs, formerly barbiturates, but today usually benzodiazepines, which depress the central nervous system (the brain and spinal cord). Non-prescription antihistamines, which are commonly marketed as sleep aids, are also available. Today, sleeping pills are generally prescribed only on a temporary basis and only if symptoms of insomnia are severe enough to seriously impede a patients life. It is possible habitual consumption of depressants to aid sleep leads to drug dependence. As with most depressents, drug tolerance may develop, as well as many long term side effects; it can cause paradoxical insomnia.

Stimulant drugs
Conversely, drugs may also be used to dissuade or delay sleep. The stimulant modafinil reduces drowsiness and is prescribed to treat narcolepsy. It allows the user to cope with shorter sleep periods while producing only minimal euphoric effects. The amphetamine family of stimulants by comparison would help to keep people awake longer but preclude the opportunity for a given person to obtain "recovery sleep" for a longer time. Modafinil, while maintaining a longer elimination half-life, does seem to allow sleep earlier than amphetamines. Simply put, modafinil removes the urge to fall asleep, rather than encouraging wakefulness. Modafinil also appears to reduce, but not elimate, the need for recovery sleep, compared to amphetamine and placebo PMID 10607162.