Jerome Siegel of UCLA asks the baggy-eyed question: why do animals need sleep? Why does anyone need sleep? Why do different species vary so widely in the amount of sleep they need? Here's his evolutionary theory:

"The analogy I make is between hibernation and sleep," [Siegel] said. "No one says, 'What is hibernation for? It is a great mystery.' . . . It's obvious that animals hibernate because there is no food, and by shutting down the brain and body they save energy." Sleep, Siegel suggested, may play much the same role. As evidence, he cited research that has found systematic differences in the way carnivores, omnivores and herbivores sleep: Carnivores sleep longer; herbivores, shorter; and omnivores, including humans, are somewhere in the middle.

"If animals have to eat grass all day, they can't sleep a lot, but if they eat meat and are successful at killing an antelope, why bother to stay awake?" he asked.

On the other hand, mammals at greater risk of being eaten -- such as newborns -- spend large amounts of time asleep, presumably safe in hiding places devised by their parents. Supporting the evolutionary explanation, Siegel's own research has shown that when the luxury of safe hiding places is unavailable -- in the ocean, for instance -- baby dolphins and baby killer whales reverse the pattern found among terrestrial mammals. These marine mammals sleep little or never as newborns and gradually increase the amount they sleep as they mature.

So animals doze off because they haven't got much else going on. Okay. But why is there no override? And why is it so painful to avoid sleeping if, historically, we've only done it because, "Hey, why the hell not? No food here..." Maybe the next great leap in human productivity will come when scientists figure out how to let us forego sleep. That could be exciting. Or maybe extraordinarily violent, what with people forced to be around each other all the time. It's hard to say how much of human society is the way it is because we need to sleep for eight hours a night. In the sleepless future, maybe, people won't even need homes; or at least, it's not clear that we would have developed the concept of a "home" in the first place if we didn't need sleep. Or maybe it's just good to have a place to do laundry. Hmm, must go find coffee now...