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 prey 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 general 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 function of sleep is to decrease it? Wilse Webb of 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 quiet 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
睡眠是古老的.從腦電圖上看，我們人類和所有靈長目動物以及幾乎所有的哺乳動物 和鳥類都一樣需要睡眠;甚至爬行類動物也有睡眠.
有證據顯示，有夢睡眠和無夢睡眠這兩種類型的睡眠取決於該動物的生活方式。從統計上看，食肉動物比被捕食動物有更多的有夢睡眠，而被捕食動物更多地無夢睡眠。 動物在有夢睡眠時，被有效地解除動作能力， 並且對外界刺激缺乏反應。無夢睡眠則要淺得多.我們都看到過貓和狗在顯然的酣睡中，有一點響動耳朵就會豎起來。被捕食動物很少有深度的有夢睡眠，這看來顯然是自然選擇的結果.而且這一點是有道理的：當睡眠高度進化以後，愚笨的動物比聰明的動物更少在深度睡眠狀態下喪失動作能力。
但是動物為什麼要進入深度睡眠呢?為什麼這樣的無動作狀 態也會進化出來呢? 海豚、鯨魚以及水生哺乳動物睡眠都極少，這一事實可以給睡眠的根本功能提供有用的線索. 海洋中是沒有藏身之處的。 會不會是這樣，睡眠不但不增加動物受傷害的可能性，反而是減少了這種可能性呢?佛羅裡達大學的 Wilse Webb 和倫敦大學的 Ray Meddis 認為情況就是如此。可以想像得出，在危險的時刻，那些由於太愚笨而不能自動保持安靜的動物，會不由自主地變得動彈不得。這一點在食肉動物的幼獸身上表現得特別明顯。這是一個很有意思的看法，它至少部分是正確的。