Evolution of Sleep

　　Sleep is very ancient. In the electroencephalographic sense we share it

　　with all the primates and almost all the other mammals and birds: it may

　　extend back as far as the reptiles.

　　There is some evidence that the two types of sleep, dreaming and dreamless,

　　depend on the life-style of the animal, and that predators are

　　statistically much more likely to dream than prey, which are in turn much

　　more likely to experience dreamless sleep. In dream sleep, the animal is

　　powerfully immobilized and remarkably unresponsive to external stimuli.

　　Dreamless sleep is much shallower, and we have all witnessed cats or

　　dogs cocking their ears to a sound when apparently fast asleep. The fact

　　that deep dream sleep is rare among prey today seems clearly to be a

　　product of natural selection, and it makes sense that today, when sleep is

　　highly evolved, the stupid animals are less frequently immobilized by

　　deep sleep than the smart ones. But why should they sleep deeply at all?

　　Why should a state of such deep immobilization ever have evolved? Perhaps

　　one useful hint about the original function of sleep is to be found in the

　　fact that dolphins and whales and aquatic mammals in general seem to sleep

　　very little. There is, by and large, no place to hide in the ocean.

　　Could it be that, rather than increasing an animal's

　　vulnerability, the function of sleep is to decrease it? Wilse Webb of the

　　University of Florida and Ray Meddis of London University have suggested

　　this to be the case. It is conceivable that animals who are too stupid

　　to be quiet on their own initiative are, during periods of high risk,

　　immobilized by the implacable arm of sleep. The point seems particularly

　　clear for the young of predatory animals. This is an interesting notion and

　　probably at least partly true.