It's an interesting choice from an evolutionary standpoint: At the time homo sapiens evolved, we presumably had saber-tooth tigers, tyrannosaurus rexes, abonimable snowmen (or whatever :) prowling the bushes around the place we slept.

If you were all alone, wouldn't it be a better survival trait to be groggy 24h/day than to be fully alert 16h/day and dead to the world the remaining 8h?

I've heard that there are species of fish (and possibly birds) that are awake around the clock; they deal by sleeping one brain hemisphere at a time, which leaves enough active brain to avoid obstacles and predators.

If humans never took that evolutionary road, does it mean that we are genetically disposed to social groups, i.e. "someone else is watching your back while you sleep"?