sleet is part of a person's daily activity cycle. there are several different stages of sleep, and they too occur in cycles. if you are an average sleeper, your sleep cycle is as follows. when you fist drift off into slumber, your eyes will roll about a bit, you temperature will drop slightly, your muscles will rela, and your breathing well slow and become quite regular. your brain waves slow and become quite regular. your brain waves slow down a bit too, with the alpha rhythm of rather fast waves 1 sleep. for the net half hour or so, as you rela more and more, you will drift down through stage 2 and stage 3 sleep. the lower your stage of sleep. slower your brain waves will be. then about 40to 69 minutes after you lose consciousness you will have reached the deepest sleep of all. your brain will show the large slow waves that are known as the delta rhythm. this is stage 4 sleep.
you do not remain at this deep fourth stage all night long, but instead about 80 minutes after you fall into slumber, your brain activity level will increase again slightly. the delta rhythm will disappear, to be replaced by the activity pattern of brain waves. your eyes will begin to dart around under your closed eyelids as if you were looking at something occurring in front of you. this period of rapid eye movement lasts for some 8 to 15 minutes and is called rem sleep. it is during rem sleep period, your body will soon rela again, your breathing will slip gently back from stage 1 to stage 4 sleep----only to rise once again to the surface of near consciousness some 80 minutes later.
第二篇:睡眠演变的英语作文
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 etend 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 eperience dreamless sleep. in dream sleep, the animal is powerfully immobilized and remarkably unresponsive to eternal 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.