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Interesting point on the evolutionary conundrum of sleep. I'm not sure I follow the teleological argument, or if I even like teleological arguments, but here's my two pesos on the evolution of sleep:

You are certainly more vulnerable when you sleep, and death is a particularly strong selective force, so it would seem quite obvious that sleeping would be selected against. On the flipside, the advantages of sleeping are hightened levels of performance during wake time. Imagine if you had a mechanic living in your garage who would fix your car every night. You could drive it like a maniac during the day, knowing that at night, your little mechanic/elf would make your car nice and new the next morning. For an animal, the advantage is that the body can focus energy on being active during periods of activity, and focus energy on repair during periods of sleep.

While the disadvantage of being killed in one's sleep is more of a direct problem for prey than for a predator, both parties benefit from sleeping. The predator is a better hunter after a long rest and the prey is more likely to survive if chased after a refreshing nap. Still, extra energy alone isn't enough of a selective advantage for sleep. If energy for activty was the only necessity, why wouldn't a predator reverse sleeping schedule and become nocturnal, while its diurnal prey is napping?

My hypothesis is that the ability to hunt sleeping prey confers too large of a short-term advantage to be stable for a predator-prey relationship. Predator-prey relationships in nature are to some degree symbiotic. Most nocturnal predators hunt nocturnal prey and diurnal predators hunt diurnal prey. If field mice slept out in the open plains so that barn owls could browse around and gobble them up, mice would quickly disappear from that field. But, once all the mice were gone, the owls, if they had no other diet, would die off soon after from starvation. So, if owls evolved the ability to stay awake 24 hours and catch mice at day while they sleep, sure those owls will be fat and happy, but soon they will quickly outstrip their resources and die.

This argument is teleogical only to the extent that it invokes ideas of Gaia Theory where the real selection is not for survival of an individual, or for a species, but for survival of an ecosystem. An ecosystem that has complementary predator-prey relationships is more likely to be stable in the long run.

One more analogy (evil things, analogies) are rules between humans concerning war. Why is it that there are bans on biological weapons? Why is it that armies would cease fighting at night and sometimes even fraternize? Because, being 24 hour and merciless costs a lot of energy. Because using biological weapons threatens the entire species. Maybe this issue should be investigated more based on game theory rather than evolutionary biology. Since I have studied neither very extensively, I hope this speculation I've presented isn't too asinine.