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 pray 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 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 vulnerability, 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 quite 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.
睡眠的进化
睡眠是非常古老的。从脑电图上看，我们与所有的灵长类动物，几乎所有的哺乳动物和鸟类可以追溯到爬行动物。
有一些证据表明，这两种类型的睡眠，做梦和无梦睡眠取决于该动物的生活方式，和食肉动物在统计上是更容易做梦超过猎物，从而更可能体验到无梦的睡眠。在梦睡眠中，动物被有力地固定，对外界刺激反应迟钝。无梦的睡眠是非常浅，我们都看到过猫或狗就会竖起耳朵的声音明显在熟睡。在今天的祈祷中，深梦睡眠是罕见的事实，这似乎是自然选择的产物，而今天，当睡眠高度进化时，愚蠢的动物往往比聪明的人更难被沉睡所代替。但是他们为什么要睡得很深呢？为什么这种深层固定的状态会进化？
也许关于睡眠的原始功能的一个有用的暗示是海豚和鲸鱼以及水生哺乳动物似乎很少睡觉这一事实。总的来说，没有地方可以藏在海里。也许，而不是增加一种动物的脆弱性，佛罗里达大学和伦敦大学的Ray Meddis提出了这种情况。可以想象的是，动物谁是太愚蠢是完全主动的，在高风险的时期，会不由自主地变得动弹不得。这一点对于食肉动物的年轻人来说尤其明显。这是一个有趣的想法，也许至少有一部分是真的。
