Most scientists think that sleep does something important something vital for life, although research has not yet identified specifically what sleep does. Nevertheless, we all know when we need to sleep we can feel this need. We also know when sleep has done its work we feel rested and that we have slept enough. Another important feature of normal sleep is that it can end quickly. Although a sleeper may appear to be unconscious unlike someone who is actually knocked out, anesthetized or in a coma a sleeping person can easily be awakened and can resume normal waking activity within a minute or two.
Sleep is an active, highly organized sequence of events and physiological conditions.
The NREM and REM types of sleep are as different from one another as both are different from wakefulness. NREM Sleep NREM sleep is further divided into stages 1-4, based on the size and speed of the brain waves generated by the sleeper. Stages 3 and 4 of NREM sleep have the biggest and slowest brain waves. These big, slow waves are called delta waves, and stages 3 and 4 combined are often called slow-wave sleep or delta sleep . During REM sleep you can watch the sleeper s eyes move around beneath closed eyelids.