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There are five periods of sleep. One of these periods is called as REM and the other one is non-REM. Non-REM period in itself is divided into two main sections. The first one is light sleep (1st and 2nd periods) and deep sleep (3rd and 4th periods)
Sleep can be defined as periodic transitions between five sleep periods and wakefullness. After a short period of wakefullness, people usually enter the first, second, thirth and fourth periods. Light sleep creates a transition period between wakefullness and sleep and people can be easily wake up in this period. Whereas, a more intense stimuli is needed to wake up people in deep sleep. Sleep is not only slowing down the organism, but also is an active and cyclical process. It is known that brain works during sleep and the information is stored.
An adult person needs 7-8 hours of sleep a day but this changes from person to person. There are kinds of sleep disorders. These are insomnia, hipersomnia and narcolepsy. It is very difficult for the person who is insomnia to fall asleep. The patient cannot sleep for hours even succeeds this wakes up very early and feels tired. Hipersomnia is excessive sleep. Although the patient sleeps well at night, it goes on daytime. This prevents the daily routine works.  is sudden sleep. The patient can sleep at any time such as while driving, writing a letter or talking with other people.