Sleep 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 first drift off into slumber, your eyes will roll about a bit, your temperature will drop slightly, your muscles will relax, and your breathing well slow and become quite regular. Your brain waves slow down a bit too, with the alpha rhythm of rather fast waves predominating for the first few minutes. This is called stage 1 sleep.
For the next half hour or so, as you relax more and more, you will drift down through stage 2 and stage 3 sleep. The lower your stage of sleep, the slower your brain waves will be.
Then about 40 to 60 minutes after you lose consciousness you will have reached the deepest sleep of all. Your brain waves will show the large slow waves that are known as the delta rhythm. This is stage 4 sleep.
大约在开始睡眠后的 40 到 60 分钟，你将进入沉睡状态。这时的脑电波表现为巨大的缓波，被称为δ节奏。这就是第四阶段睡眠。
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 relax again, your breathing will grow slow and regular once more, and you 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.
但你并不是整夜都保持这种沉睡状态。入睡后约80分钟左右，你的大脑运动水平会再度略有提高。δ节奏消失，并被脑电波的运动图形取代。 你的眼睛会在闭着的眼睑下迅速转动，就好象你在看着眼前发生的什么事情。这种迅速的眼球运动持续约 8～15 分钟，这一阶段睡眠被称之为快速眼动(REM)睡眠。在 REM 睡眠阶段，你的肢体会很快再度放松，呼吸也再次放慢并变得有节奏，你会轻松地从第一阶段滑入第四阶段睡眠-直到大约 80分钟后重新接近清醒状态。
alpha ：n. 脑波
rhythm ：n. 节奏;韵律
delta：n. (脑电图的)δ波，(表示处于最深的睡眠状态的) 脑波