Explain different stages of sleep.

            Research on thousands of volunteers indicates that sleep can be divided into four stages. The transition from wakefulness to sleep occurs with the onset of stage 1 sleep. During this stage, a mixed but relatively slow, low-voltage EEG pattern emerges. Breathing slows, muscle tone decreases and the body generally relaxes. At this point, individuals can still be readily awakened by external stimuli. If they are not awakened they move to stage 2. During this stage the brain emits occasional short bursts of rapid, high-voltage waves known as sleep spindles. During this stage, sleepers are much more difficult to awaken than they were during stage 1. Stage 2 is followed by stage 3 and 4 which are marked by the increasing appearance of slow, high voltage delta waves and by a further slowing of all major bodily functions.

            About ninety minutes after the process begins, most persons enter a highly distinct phase known as Rapid Eye Movement (REM) sleep. During this phase, the electrical activity of the brain changes rapidly, so that it now closely resembles that shown when people are awake. Delta waves disappear, and fast, low-voltage activity returns. Sleeper’s eyes start to move about rapidly beneath their closed eyelids. Their relaxation is so great that a state bordering on paralysis seems to exist. At the same time, males may experience erections and females corresponding changes in their sexual organs. This combination of signs of activation along with signs of profound relaxation has led some researchers to describe REM sleep as paradoxical in nature, and in several respects, this description seems apt.

            Dreams are mostly found to happen during REM sleep. In some cases, eye movements during such sleep seem to be related to the content of the dreams. It is as if the individual following the action in a dream with his or her eyes.

             Stages of sleep keep changing and REM sleep happens 3-4 times in a single night. Duration of REM sleep tends to increase towards morning and that of deep sleep decreases.