Sleep is a behavioural state characterized by little physical activity and almost no awareness of the outside world. 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 be easily awakened and can resume normal waking activity within a minute or two.
Sleep is an active, highly organized sequence of events and physiological conditions. Sleep is actually made up of two separate and distinctly different states: 'non-rapid eye movement sleep (NREM sleep) and 'rapid eye movement sleep' (REM sleep) or dreaming sleep. The NREM and REM types of sleep are as different from one another as both are different from wakefulness.
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 sleep 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. Some scientists think that the eyes move in a pattern that relates to the visual images of the dream. We are almost completely paralyzed in REM sleep -- only the heart, diaphragm, eye muscles and the smooth muscles (such as the muscles of the intestines and blood vessels) are spared from the paralysis of REM sleep.
Doctors have tried to determine what type of sleep is the deepest sleep. To do this, they measure how much noise or other alerting stimulation is required to awaken a sleeper from the various types of sleep. It is always possible to awaken someone who is sleeping, as opposed to, say, someone who is in a coma. However, people in stages 3 and 4 sleep require the most stimulation to awaken. Therefore, this phase of sleep is often thought of as 'deep sleep'. Also, large spurts of growth hormone are secreted during stages 3 and 4 NREM sleep. Consequently, these stages of sleep are thought to restore the body from the wear and tear of waking activity.
People in REM sleep also tend to be quite difficult to awaken, but this finding is variable -- sometimes even the slightest noise can awaken a person in REM sleep. Nevertheless, because it is often difficult to awaken a person from REM sleep, many doctors think also of REM sleep as a 'deep' phase of sleep.
There are many theories about the function of REM sleep and dreaming -- ranging from 'safe, socially acceptable, wish fulfilment' to 'consolidation of memories' to 'providing necessary stimulation to the entire nervous system during development'. Researchers used to think that REM sleep was necessary for normal psychological function, because experimental REM deprivation caused some subjects to behave strangely. The notion that we need REM sleep for our mental health is not accepted now, because, among other reasons, people have uneventfully withstood long and almost complete REM deprivation. Some experiments have shown that REM deprivation improves depression. However, REM sleep must still do something, because rats will die after 2 - 3 weeks if they are deprived of REM sleep by a special experimental computer that wakes them up each time REM sleep is achieved. Whatever REM sleep does, it is clear that every aspect of existence, from the body's manufacture of proteins to sexual arousal including orgasm, is influenced by REM sleep. It is likely that the ultimate explanation of REM sleep will be very broad -- not simply focused on one physiologic function