Sleep is very ancient. In the electroencephalographic sense we share it with all the primat。es 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 rema。rkably。 unresponsive to external sti。muli. Dreamless s。leep is much shallower, and we have all witness。ed cats or dogs cocking their ears to a sound when apparently fast asleep. The fact that deep dream sleep is rare among pray today seems clearly to be a product of natural selection, and it makes sense that today, when sleep i。s highly evolved, the st。upid animals are less frequently immob。ilized by deep sleep than th。e smart ones. But why should the。y sleep deeply at all? Why should a state of such deep im。mobiliza。tion ever have evolved?
Perhaps one useful hint about t。he original function of sleep is to be found in the fact that dolphins and whales and aquatic mammals i。n genera 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 vulne。rability, the University of Florida and Ray Meddis of London University have suggested this to be the case. It i。s conceivable that animals who are too stupid to be quite on their own initiative are, during periods of high risk, immobilized by the implacable arm of sleep. The point seems particularly clear。 for t。he young of predatory animals. This is an interesting notion and probably a。t least partly true.