It’s such a commonplace event that you probably never asked yourself why you sleep. But from a scientific point of view, this is far from a mundane matter — in fact, for sleep researchers, it’s one of the biggest unanswered questions in the field. Stop and think about it for a second. Why should we sleep? Is it because we get tired? But we sleep every night, even when we have had plenty of rest. Furthermore, if you were designing an animal, would you have it lapse into prolonged periods of unconsciousness every twenty-four hours? Shouldn’t that put it at a terrible disadvantage? There are no doubt several different answers to this question, but let’s just consider one overall purpose sleep may fulfill: sleep as a survival strategy.