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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 once every 24 hours even though the average human inner body clock usually runs a 24.5-25.5 hour cycle. The cycle is reset daily (that is, every 24 hours) by various stimuli, the level of sunlight being the most obvious example. One of the correlates of this cycle is the level of melatonin, which is high at times when we tend to sleep.

In some cultures it is common to sleep twice every 24 hours (afternoon nap, siesta).

Sleep also refers to the mucus that collects around the eyes during sleep as defined above.

1 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 other states that share this property. As such, there are two characteristics that separate real sleep from such states such as coma or death:

Sleep is reversible. A sleeping person can be awakened.
External stimuli (sound, touch, etc.) can be incorporated into dreams.

1.1 Stages of sleep

Human sleep is usually divided into 5 stages according to electroencephalographic (EEG) recordings:

REM sleep with rapid eye movements
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
Stage 2 with "spindles" (12-16Hz) and "K-complexes"
Stage 3 with delta waves (1-2Hz) 20%-50% of the time.
Stage 4 with delta waves over 50% of the time

2 Animal sleep

Sleepy lions Animals vary widely in their amounts of sleep, from 2 hours a day for giraffes to 20 hours for batThis article is about mammals. For other bats, see bat (disambiguation) and bats. Pteropodidae Emballonuridae Rhinopomatidae Craseonycteridae Rhinolophidae Nycteridae Megadermatidae Vespertilionidae Molossidae Antrozoidae Natalidae Myzopodidae Thyropterids. Generally, required sleeping time decreases as body size increases. CatFor alternative meanings see cat (disambiguation). The cat (also called domestic cat or house cat is a small feline carnivorous mammal. Its scientific name is Felis silvestris catus or Felis silvestris domesticus but the species is sometimes referred to as 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 dolphinThis article is about the dolphin mammal. For other uses of the term, please see dolphin (disambiguation). See article below. Dolphins are certain aquatic mammals related to whales and porpoises. The word is used in a few different ways. It can mean: #anys "sleep" with alternate hemispheres of their brainsFor other articles about other subjects named brain see brain (disambiguation). In the anatomy of animals, the brain or encephalon is the supervisory center of the nervous system. Although the brain is usually cited as the supervisory center of vertebrate asleep and the other awake. Seals need to do this so they can breathe above water while sleeping.

Even fishAtlantic herring, Clupea harengus one of the most abundant species in the world Photo A fish is a poikilothermic (cold-blooded) water-dwelling vertebrate with gills. There are over 27,000 species of fish, making them the most diverse group of vertebrates. 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 hibernateHibernation is a state of regulated hypothermia, lasting several days or weeks that allows animals to conserve energy during the winter. During hibernation animals slow their metabolism to very low levels, with body temperature and breathing rates lowered 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.

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