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Sleep is the process in which humans and other animals periodically rest, with greater or lesser degrees of unconsciousness and decreased responsiveness to the surrounding world. Sleep occurs cyclically, roughly every 24 hours even though the average human inner body clock usually runs a 24.5-25.5 hour cycle. This cycle gets reset daily (to match 24 hours) with various stimuli such as sunlight. One of the correlates of this cycle is the level of melatonin, which is high at times when we tend to sleep. Some people sleep twice every 24 hours (afternoon nap, siesta).

Table of contents
1 Characteristics of sleep
2 Animal sleep
3 Functions of sleep
4 Sleep disorders
5 Sleepiness
6 See also
7 External links

Characteristics of sleep

During sleep, one loses consciousness of one's surroundings. The sleeper is not directly aware of the outside world.

However, there are two characteristics that separate real sleep from many things that are described as "sleep", but really are not, such as coma or death:

  1. It is reversible. A sleeping person can be awakened.
  2. sounds, etc. can be incorporated into dreams.

Stages of sleep

Human sleep is usually divided into 5 stages according to
electroencephalographic (EEG) recordings:
  1. REM sleep with rapid eye movements
  2. Stage 1 with 50% reduction in alpha waves compared to awake resting with eyes closed. The stage is sometimes referred to as somnolence or "drowsy sleep". It appears at sleep onset and can be associated with so-called hypnagogic hallucinations
  3. Stage 2 with "spindles" (12-16Hz) and "K-complexes"
  4. Stage 3 with delta waves (1-2Hz) 20%-50% of the time.
  5. Stage 4 with delta waves over 50% of the time

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.

Seals and dolphins "sleep" with alternate hemispheres of their brains asleep and the other awake. Seals need to do this so they can breathe above water while sleeping.

Even fish and fruit flies appear to sleep. If fruit flies are repeatedly disturbed so that they can not sleep, later when allowed to sleep they will stay inactive for a longer period of time.

Many animals hibernate in a deep sleep during winter to save warmth and energy. A similar kind of sleep is estivation, which is hibernating to escape the heat of summer.

Functions of sleep

Though there is still much debate about the evolutionary origins and purposes of sleep, it is widely theorised that one major function that occurs during sleep is consolidation and optimization of memories (including "unlearning").

Previously theorized functions that do not seem to be confirmed by research:

Sleep proceeds in cycles of NREM and REM phases. Each phase has a distinct physiological function. Dreaming, for example, appears to occur during REM sleep.

Adenosine, a nucleoside which plays various roles in biochemical processes, gradually accumulates in the human brain during wakefulness but decreases during sleep.

Some drugs, such as alcohol and sleeping pills, can suppress certain stages of sleep. This can result in sleep, that is loss of consciousness, without fulfilling its physiological functions.

A lack of sleep can lead to impaired functioning. If the insufficiency is small, a sleep debt can accumulate, leading to drowsiness. Severe lack of sleep, up to and including a total lack of sleep, can have quite severe psychological effects. Human subjects have been observed going without sleep for up to eleven days; during the last part of their ordeals, they were effectively non-functional. Experiments with rats have been designed to measure the effects of severe sleep deprivation. In one, a pair of rats was placed on a platform, 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, could sleep in between movements. After several weeks of this, the subject rats became unable to regulate body temperature; even if they were allowed to sleep at this point, they died shortly afterward.

Sleep disorders

A majority of sleep disorders which originate within the body (for example, insomnia, Delayed Sleep Phase Syndrome, or Advanced Sleep Phase Syndrome) result from errors in synchronization of sleep with the body clock. Only a fraction of sleep problems are organic and cannot be resolved with chronotherapy. One of the simplest solutions towards getting good sleep is 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 usually cannot be employed due to the resulting loss of synchronization of sleep with the outside world (including day-night cycle).

Sleep disorder is often observed in patients with a number of psychiatric problems (e.g. bipolar disorder, depression, schizophrenia, etc.).

One form of sleep disorder, narcolepsy, probably has a genetic basis. Subjects not only fall asleep several times during the day, they also experience abnormal sleep patterns at night. A new medication is Xyrem, the proprietary name of gamma-hydroxybutyrate (GHB). In the United States of America, it has recently been approved by the Food and Drug Administration.

Sleep can also refer to the state of hypnosis.


Sleepiness normally occurs when it is time to sleep, but it can e.g. also be caused by a sedative.

See also

External links


Nap is also a betting term for the best bet on a particular day.
NAP also stands for Normaal Amsterdams Peil.