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.
Human beings are "diurnal" creatures. That means we are normally active during daylight hours, when our senses function most effectively. At night, humans do rather poorly: we can't see objects well, our color vision is entirely lost, and we don't have the smelling or hearing acuity of other animals. So it actually does makes sense to have us stay put during the dangerous period when "nocturnal," or night-waking, animals are on the prowl. And one sure way to make sure we don't stumble around and get lost or eaten is to have us immobilized for seven or eight hours, rising again only when the light is back and our survival chances are better.
It's not the only reason we sleep; but in terms of evolution, it may have been one of the first reasons -- perhaps even the most important one.
