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 relax， 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 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. 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 relax 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.