Sleep: Encyclopedia II - Sleep - Animal sleep

Sleep - 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.

Adapted from the Wikipedia article "Animal sleep", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki