Why sleep?

Here's a simple and attractive answer, from Circadiana:

...sleep makes you sit still and be quiet at times when it is dangerous to move around and there is nothing else important to do.

None of that recharging mental batteries stuff (insects, which have "the mentative power of a flashlight", sleep); none of that recharging physical batteries stuff (lazy people need as much sleep as atheltic people). Nope, sleep might have developed as nature's way of telling our ancestors to sit (lie) down and shut up. For an extreme example, think of hibernation - no need for bears to be out running around all winter long - just sleep through it! The fact that critters now do all kinds of other things during sleep is just because nature took advantage of the down time sleep brought to perform a little maintenance.

Hooray for biology.