Like hunger and thirst, sleep is a basic biological drive we can't ignore. In fact, when we don't get enough of it, our bodies eventually force us to make up for the sleep lost. However, why we sleep remains a mystery–and one of the biggest unanswered questions in psychology.

But recently, results from sleep studies in humans and animals have begun to lead some promising explanations. Researchers are finding that sleep may help babies learn the placement of their own limbs ("To sleep, perchance to twitch"), it may help adults fix new memories in their brains ("Let's sleep on it"), it may give the brain time to replenish energy stores ("Brain, heal thyself") or it may allow it to recuperate from the learning it does during the day ("Wild findings on animal sleep"). Or–perhaps most likely–sleep may do some combination of all those things.

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