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.

 

睡眠的進化

 

睡眠是非常古老的。從腦電圖上看，我們與所有的靈長類動物，幾乎所有的哺乳動物和鳥類可以追溯到爬行動物。

 

有一些證據表明，這兩種類型的睡眠，做夢和無夢睡眠取決于該動物的生活方式，和食肉動物在統計上是更容易做夢超過獵物，從而更可能體驗到無夢的睡眠。在夢睡眠中，動物被有力地固定，對外界刺激反應遲鈍。無夢的睡眠是非常淺，我們都看到過貓或狗就會豎起耳朵的聲音明顯在熟睡。在今天的祈禱中，深夢睡眠是罕見的事實，這似乎是自然選擇的產物，而今天，當睡眠高度進化時，愚蠢的動物往往比聰明的人更難被沉睡所代替。但是他們為什么要睡得很深呢？為什么這種深層固定的狀態會進化？

 

也許關于睡眠的原始功能的一個有用的暗示是海豚和鯨魚以及水生哺乳動物似乎很少睡覺這一事實�？偟膩碚f，沒有地方可以藏在海里。也許，而不是增加一種動物的脆弱性，佛羅里達大學和倫敦大學的Ray Meddis提出了這種情況�？梢韵胂蟮氖牵瑒游镎l是太愚蠢是完全主動的，在高風險的時期，會不由自主地變得動彈不得。這一點對于食肉動物的年輕人來說尤其明顯。這是一個有趣的想法，也許至少有一部分是真的。