Test Your Knowledge

Sleep disorders are disturbances of usual sleep patterns or behaviors.

Sleep disorders may involve:

• Difficulty falling asleep or staying asleep
• Falling asleep at inappropriate times
• Excessive total sleep time
• Abnormal behaviors during sleep

Researchers have studied the sleep cycle for more than 50 years. It is not entirely understood. Chemical substances called neurotransmitters control sleep and wakefulness by affecting neurons (nerve cells) in the brain. Neurotransmitters, such as serotonin and norepinephrine, keep parts of the brain active while awake. When sleep begins, neurons in the base of the brain "switch off" the signals that maintain wakefulness.

Researchers measure sleep based on the electrical activity of the brain and other parts of the body. Sleep can be divided into five stages:

• Stage 1. Light transitional sleep. People are drowsy and drift in and out of sleep. They awaken readily from this stage. When awakened, people may remember bits of visual images. People may experience sudden muscle contractions (hypnic myoclonia) often preceded by a sense of falling. Stage 1 represents about 5 percent of sleep time in a normal sleep cycle. People with some sleep disorders have more periods of stage 1 sleep.
  
  
• Stage 2. Eye movement stops and brain waves (fluctuations of electrical activity) become slower, with occasional bursts of rapid waves (sleep spindles). About 50 percent of sleep is stage 2 sleep.
  
  
• Stage 3. Dramatically slower brain waves (delta waves) begin to appear along with smaller, faster waves.
  
  
• Stage 4. Delta waves predominate during this stage. Stages 3 and 4 are called deep sleep. It is difficult to wake people during these stages. When awakened during this stage, people often feel disoriented and groggy. Children sometimes experience bedwetting, night terrors or sleepwalking during deep sleep.
  
  
• REM (rapid eye movement) sleep. Breathing becomes more rapid, irregular and shallow and the eyes move rapidly in various directions. Limbs become temporarily paralyzed. Heart rate and blood pressure increase and males develop erections. People often dream during REM sleep.

Sleep progresses in a cycle from stage 1 through REM sleep. A complete sleep cycle takes an average of 90 to 110 minutes. The first period of REM sleep usually occurs about 70 to 90 minutes after falling asleep. The first sleep cycle each night contains relatively short REM periods and long deep sleep periods. As sleep progresses, REM sleep periods increase and deep sleep decreases. By the end of the sleep period, people spend nearly all their time in stages 1, 2 and REM sleep.

Although sleep is necessary to survival, sleep requirements vary. Infants generally require about 16 hours of sleep a day and teenagers need about 9 hours. Most adults require 7 to 8 hours, although daily requirements may range from 5 to 10 hours. Women in the first three months of pregnancy often need several more hours of sleep than usual. As people age, they tend to sleep more lightly and for shorter periods of time, although they generally need about as much sleep as they did in early adulthood.

Sleep disorders are extremely common, but many are undiagnosed and untreated. More than 70 million Americans have a sleep disorder, according to the National Institutes of Health. Among those, almost 60 percent have a chronic disorder.

Sleep disorders affect people of all ages, including about 25 percent of children between the ages of 1 and 5 years, according to the National Center on Sleep Disorders Research (NCSDR). Of all the age groups, infants and elderly people are most likely to experience problems with sleep. About half of people over age 65 have frequent sleep problems. Sleep disturbances in the elderly may be a normal part of aging, a result of medical or psychiatric problems or a side effect of the increased intake of medications used to treat these problems.

The incidence of some sleep disorders increases with age, and with the number of older Americans increasing, it is estimated that nearly 80 million Americans will have a sleep problem by 2010 and 100 million will have one by 2050, according to the NCSDR.

Sleep disorders, sleep deprivation and sleepiness result in billions of dollars in health costs each year in the United States. Other impacts include reduced worker productivity and automobile accidents. Sleep disorders have also been linked to obesity, heart disease, strokes, diabetes and other health problems.