Sleep disorders are a group of syndromes characterized by disturbance in the individual's amount of sleep, quality or timing of sleep, or in behaviors or physiological conditions associated with sleep.
Although sleep is a basic behavior in animals as well as humans, researchers still do not completely understand all of its functions in maintaining health. Since 1975, however, laboratory studies on human volunteers have yielded information about the different types of sleep. Researchers have learned about the cyclical patterns of different types of sleep and their relationships to breathing, heart rate, brain waves, and other physical functions. These measurements are obtained by a technique called polysomnography. There are about 70 different sleep disorders. To qualify for the diagnosis of sleep disorder, the condition must be a persistent problem, cause the patient significant emotional distress, and interfere with his or her social, academic, or occupational functioning.
There are five stages of human sleep. Four stages have non-rapid eye movement (NREM) sleep, with unique brain wave patterns and physical changes occurring. Dreaming occurs in the fifth stage, during rapid eye movement (REM) sleep.
Stage one NREM sleep. This stage occurs while a child is falling asleep. It represents about 5 percent of sleep time.
Stage two NREM sleep. In this stage, (the beginning of "true" sleep), the child's electroencephalogram (EEG) will show distinctive waveforms called sleep spindles and K complexes. About 50 percent of sleep time is stage two NREM sleep.
Stages three and four NREM sleep. Also called delta or slow wave sleep, these are the deepest levels of human sleep and represent 10 to 20 percent of sleep time. They usually occur during the first 30 to 50 percent of the sleeping period.
REM sleep. REM sleep accounts for 20 to 25 percent of total sleep time. It usually begins about 90 minutes after the child falls asleep. It alternates with NREM sleep about every hour and a half throughout the night. REM periods increase in length over the course of the night.
Sleep cycles vary with a person's age. Children and adolescents have longer periods of stage three and stage four NREM sleep than do middle aged or elderly adults. Because of this difference, the doctor needs to consider the individual's age when evaluating a sleep disorder. Total REM sleep also declines with age.
The average length of nighttime sleep varies among individuals. Most people sleep between seven and nine hours a night. This population average appears to be constant throughout the world. In temperate climates, however, people often notice that sleep time varies with the seasons. It is not unusual for people in North America and Europe to sleep about 40 minutes longer per night during the winter. Infants can regularly sleep up to 16 hours a day. The total amount of sleep declines as the infant gets older. Teenagers may actually need more sleep than slightly younger children and often sleep nine or more hours a day.
Sleep disorders are classified based on what causes them. Primary sleep disorders are distinguished as those that are not caused by other mental disorders, prescription medications, substance abuse, or medical conditions. The two major categories of primary sleep disorders are the dyssomnias and the parasomnias.