S。leep is very anci。ent. In the electroencephalographic sense we share it with all the primates and alm。ost 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, dre。aming and dreamless, depe。nd on the life-st。yle o。f t。he animal, an。d that predators are statistical。ly 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 unrespo。nsive to external stimuli. Dreamless sleep is much shallower, and we have all witness。ed cats or dogs cocking thei。r e。ars to a sound when appar。ently fas。t asleep. The fact that deep dream sleep is ra。re am。ong pray today seems clearly to be a product of na。tural se。lection, and it makes sense that today, when sleep is highly evolve。d, the stupid animals are less frequently immobilized by deep s。leep 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 hi。nt about the orig。inal function of sleep is to be found in th。e fact that dolphins a。nd whales and aquatic。 mammals in genera seem to sleep very littl。e. There is, by and large, no place to hide in the ocean. Could it be that, rather than increasing an animal's vulnerabili。ty,。 the University of Florida and Ray Meddis of Lon。don University ha。ve sug。gested this to be th。e case. It is conceivable that anim。als who are too st。upid to be quite on their own initiative are, during periods of high risk, im。mobilized by the implacable arm of sleep. The point seems particularly clear for the young of predatory ani。mals. This is an interesting not。ion and probably at least partly true.