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睡眠革命

Evolution of sleep

Sleep is very ancient. In the electroencephalographicsense we share it withall the primates and almost allthe other mammals and birds: it may extend backasfar as the reptiles.

There is some evidence that the two types of sleep,dreaming and dreamless,depend on the life-style ofthe animal, and that predators are statisticallymuchmore likely to dream than prey, which are in turnmuch more likely toexperience dreamless sleep. Indream sleep, the animal is powerfully immobilizedand remarkably unresponsive to externalstimuli. Dreamless sleep is muchshallower, and we have all witnessed cats or dogs cocking theirears to a soundwhen apparently fast asleep. The fact that deep dream sleep is rare amongpraytoday seems clearly to be a product of natural selection, and it makes sensethat today,when sleep is highly evolved, the stupid animals are less frequentlyimmobilized by deep sleepthan the smart ones. But why should they sleep deeplyat all? Why should a state of such deepimmobilization ever have evolved?

Perhaps one useful hint about the original function of sleep is to be foundin the fact thatdolphins and whales and aquatic mammals in genera seem to sleepvery little. There is, by andlarge, no place to hide in the ocean. Could it bethat, rather than increasing an animal’svulnerability, the University of Floridaand Ray Meddis of London University have suggestedthis to be the case. It isconceivable that animals who are too stupid to be quite on their owninitiativeare, during periods of high risk, immobilized by the implacable arm of sleep.Thepoint seems particularly clear for the young of predatory animals. This is aninteresting notionand probably at least partly true.