IT is one of life's great enigmas: why do we sleep? Now we have the best evidence yet for the function of sleep - that it lets housekeeping processes take place to stop our brains becoming overloaded with new memories. As far as we know, every animal sleeps (see "Why Snooze?" below), but the reason for their slumber has remained elusive. When lab rats are deprived of sleep, they die within a month, and when people go for a few days without sleeping, they start to hallucinate and may have epileptic seizures. One idea is that sleep helps us consolidate new memories, as people do better in tests if they get a chance to sleep after learning. We know that, while awake, fresh memories are recorded by reinforcing connections between brain cells, but the memory processes that take place while we sleep have been unclear.
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