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Human body

Why do we sleep?

A girl asleepA girl asleepSleep is a state where awareness of our surroundings is reduced. Scientists call this a state of "altered consciousness". Sleep is different from states of coma (prolonged unconsciousness) or hibernation because it can be rapidly reversed, and because while we are asleep our brains display very active patterns. All animals sleep for at least some period of each day; for we humans, the time we are asleep is equivalent to one third of our lives. A well-known feature of sleep is dreaming. It seems to resemble waking life while it is in progress, but which after we wake up is revealed as a fantasy.


A man and his dog asleepA man and his dog asleep

Reasons for sleeping

Sleeping for several hours made our ancestors in prehistoric times vulnerable to attack from wild animals. So the considerable risks of this must offer some kind of advantage in return, otherwise we would have soon become extinct as a species. Helping us to become more intelligent by improving our brain power, or more physically powerful by restoring our muscles to full strength, would count as such advantages. 


Nerve impulses zipping along neurons inside the brainNerve impulses zipping along neurons inside the brainIn fact, scientists do not know for sure exactly why we sleep. There are several theories, however. One is that sleep plays an important role in the way we pick up and store memories—and shed unnecessary ones. During the day, our brains take in a vast amount of information. This needs to be processed and stored, and this happens mostly while we sleep. Overnight, some of the information we have received during the day is transferred from our short-term memory to our long-term memory, which is stronger. This is called consolidation.

Animals require different amounts of sleep each day. A little brown bat sleeps 20 hours, a cat 12.1 and a chimpanzee 9.7. Large herbivores need much less. A giraffe sleeps 4.5 hours, an African elephant 3.3 hours. Horses, including zebras, sleep part of the night standing up and part lying down—but only 2 hours in total.

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