Sleep

Sleep is the process in which humans and other animals periodically rest, with 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 Related Articles
6 Additional Resources

Characteristics of Sleep

Sleep has two characteristics:

It is reversible. A sleeping person can be awakened.
During it, one loses consciousness of one's surroundings. The sleeper is not directly aware of the outside world (although, for example, sounds can be incorporated into dreams).

These two characteristics separate real sleep from many things that are described as "sleep", but really are not, such as Coma, and Death.

Animal Sleep

Animals vary widely in their amounts of sleep, from 2 hours a day for giraffes to 20 hours for bats. Cats are one of the few animals that do not have most of their sleep consolidated into one session, prefering 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. Another function of sleep is the conservation of energy during inactivity. Other theorised functions include:

promotion of physiological processes which rejuvenate the body and the mind: some studies suggest sleep restores neurons and increases production of brain proteins and certain hormones;
unlearning during sleep prevents the brain from becoming overloaded with knowledge; and
avoidance of danger: prehistoric mankind adapted the pattern of sleeping in caves at night, because it protected humans from species physiologically suited to function well in the dark, such as saber-toothed tigers.

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.

Some medications (for example, sleeping pills) can suppress selective stages of sleep. This can result in obtaining sleep (loss of consciousness) without fulfilling its physiological function (memory remolding).

Sleep Disorders

A majority of sleep disorders which originate within the body (for example, insomnia, DSPS, ASPS) 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.).

Sleep can also refer to the state of hypnosis.