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Last updated at 10:04 BST, Monday, 12 October 2009

Why do we sleep?


9 October 2009

A recent study may have an answer to one of the greatest unsolved mysteries in science - what is the purpose of sleep? The work suggests it's actually about making animals function more efficiently in their environments.

Jon Stewart

Taking a nap on the beach


Click to hear the report:


Pythons, bats and giant armadillos are among the longest sleepers at over 18 hours a day. Human babies need 16 hours, and most of us probably feel we need around eight hours sleep to function well.

Professor Jerry Seigel from the University of California, Los Angeles, conducted a study of the sleep times of a broad range of animals and found that they vary widely. Some, like migrating birds, can survive long periods without sleeping at all. He believes that shows sleep evolved to conserve energy:

Jerry Seigel: 'It's animals that are needlessly active that will not survive, but animals that are most efficient and use their waking time to do vital functions, and are otherwise asleep that will survive.'

Sleep helps make best use of limited resources. In humans, when we're awake, our brain accounts for 20% of the energy we use when just sitting around. Sleeping also makes us less likely to get injured and less likely to be detected by predators.

Jon Stewart, BBC News


Click to hear the vocabulary:


to function well

to live and operate normally

a broad range of

many different

vary widely

are very different

migrating birds

birds that move between two different environments in response to changes in weather and food supply

to conserve

to save, to keep and protect from waste

are needlessly active

move a lot without reason or purpose


essential, most important, impossible to do without

limited resources

here, when there is relatively little food

to get injured

here, to get hurt or hurt oneself accidentally

detected by predators

noticed by those who are likely to hunt and kill you

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