We spend about a third of our life doing it. If deprived of it for too long we get physically ill. So it's puzzling that we still don't really know why it is that we sleep. On the face of it the answer seems obvious: so that our brains and bodies can rest and recuperate. But why not rest while conscious, so that we can also watch out for threats? And if recuperation means things are being repaired, why can't that take place while we are awake? Scientists who study how animals eat, learn or mate are unburdened by questions about the purpose of these activities. But for sleep researchers the big "Why?" is maddeningly mysterious.
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