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. 

    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.

    睡眠是人每天日常活動循環的一部分。 人的睡眠分幾個階段，而這些階段也是循環發生的。 如果你是一個正常的睡眠者，你的睡眠循環會這樣進行。 在你開始昏昏入睡時，你的眼睛會滾動幾下，體溫略有下降，肌肉放松，呼吸變得緩慢而有節奏。 除了開始幾分鐘比較快的α節奏外，腦電波也稍有減緩。 這被稱為第一階段睡眠。 在隨后約半小時內，你進一步放松，進入第二和第三階段睡眠。 睡眠越深入，腦電波就越緩慢。 大約在開始睡眠后的40到60分鐘，你將進入沉睡狀態。 這時的腦電波表現為巨大的緩波，被稱為δ節奏。 這就是第四階段睡眠。 但你并不是整夜都保持這種沉睡狀態。 入睡后約80分鐘左右，你的大腦運動水平會再度略有提高。 δ節奏消失，并被腦電波的運動圖形取代。 你的眼睛會在閉著的眼瞼下迅速轉動，就好象你在看著眼前發生的什么事情。 這種迅速的眼球運動持續約8～15分鐘，這一階段睡眠被稱之為快速眼動(REM)睡眠。 在REM睡眠階段，你的肢體會很快再度放松，呼吸也再次放慢并變得有節奏，你會輕松地從第一階段滑入第四階段睡眠-直到大約80分鐘后重新接近清醒狀態。