[crayz]

Well, try an experiment. Find a time of day when you are alert and well-rested, and lay still in your bed with no external entertainment for eight hours

I'm not saying the idea is correct, but your argument is sorely lacking in plausibility - creatures with complex nervous systems get bored staying still unless they lose consciousness (and actually, we spend a lot of sleep with our brains still active in dream-world). If evolution "wanted" to come up with a way to keep animals out of trouble for the part of the day they'd be vulnerable, sleep might be a reasonable and generic way of doing so

[sho]

It wouldn't be lying in bed motionless. It would be sitting in the cave talking quietly instead of sleeping. Or whatever.

I don't think that being asleep is safer than being awake, even in the exact same place. Why do you think someone always stays awake to guard the camp?

Oh well, not like I know what the hell I'm talking about. I just thought Siegel's explanations were too cute by half. Of course we evolved to sleep, there's nothing about us that didn't evolve. But the idea that the state of being asleep makes us safer from predators, and that's the reason for the selection? Doesn't pass the smell test IMO. Could be wrong. Been wrong before.