As a father of two dawn-loving children I don't get as much sleep as I used to, which makes me wonder sometimes why I crave it so much. A number of scientists who share my curiosity have turned to sleeping animals to find an answer. Sleep appears to be an ancient behavior, perhaps 600 million years old or older. But it may not exist "for" any one purpose. Instead, sleep can serve many functions, as animals are shaped by evolutionary tradeoffs. I've written an article about the evolution of sleep for tomorrow's New York Times where you can read more. (And for those interested in some of the the gorey technical details, here's an interesting new review in Nature that's free.)
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Dr Zimmer:
One possible explanation for the purpose (or one purpose) of sleep which, as Dr Lima says in your article, is " ... so stinking dangerous ... " is that something else -- also "stinking dangerous" -- is going on inside the animal's body when the animal is sleeping: cell division.
Now that we know about cellular oncogenes we know that ordinary mitosis - which occurs in humans billions of times every 24 hours - has the potential to initiate a mutational event that could lead to lethal cancer.
Enforced physical quiescence induced by our brains ensures that rigorous activity, which might interfere with mitosis, is virtually eliminated.
The fact that humans sleep most when we are undergoing rapid growth (not only in infancy but in early adolescence) and lots of mitosis is one of the facts in support of this idea.
A further hint: since my book was published researchers have found that the immune system plays a role in triggering sleep.
I wonder what role light and dark play in sleep. We live and a night versus day world. Being active during the wrong time might not be adaptive in terms of interactions with predators and even competitors. It would be interesting to look at taxa that have evolved in the absence a light cycle (deep sea, caves) and see if the sleep cycle is maintained.
Actually, sleep-wake cycle does not persist in subterranean, deep oceanic and cave creatures. Circadian rhythms of other stuff (biochemical and physiological, not behavioral) persist, though.
Well. Mostly. As in everything in biology there is a spectrum of cases. In some cave animals (some salamanders), the activity cycle can be reinstated by exposure to light-dark cycles. Animals that go out (bats, oilbirds) have strong sleep-wake cycles and there is some evidence that daily activity of bats keeps the activity cycles of some cave invertebrates (roaches) from devolving away.