Sleep disorders are a group of syndromes characterized by disturbance in a person's amount of sleep, quality or timing of sleep, or in behavior while sleeping. There are about 70 different sleep disorders.

Although sleep is a basic behavior in animals as well as humans, researchers still don't completely understand it. In the past 30 years, however, researchers have learned about the pattern of different types of sleep and its effects on breathing, heart rate, brain waves, and so on.

There are five stages of human sleep. Four stages occur during "non-rapid eye movement (NREM) sleep," featuring unique brain wave patterns and body changes. Dreaming occurs in the fifth stage, known as "rapid eye movement (REM)" sleep. Stage 1 NREM sleep begins as a person falls asleep, progressing into Stage 2 NREM sleep, the beginning of "true" sleep. Experts can identify this stage by looking at a person's brain waves...