Understanding Sleep

Sleep is a complex neurological state. Its primary function is rest and restoring the body's energy levels. Repeated interruption of sleep by breathing abnormalities such as cessation of breathing (apnea) or heavy snoring, leads to fragmented sleep and abnormal oxygen and carbon dioxide levels in the blood. Excessive daytime sleepiness and various disorders of the heart, lungs, and the nervous system result.

In the 1950's scientists realized that sleep is not just a quiet state of rest. In fact, two stages of sleep occur with distinct physiological patterns-rapid-eye-movement sleep (REM), and non rapid-eye-movement sleep (NREM) or deep sleep. In normal sleep, REM occurs about 90 minutes after a person falls asleep. The two sleep stages recur in cycles of about 90 minutes each, with three non-REM stages (light to deep slumber) at the beginning and REM towards the end. The amount of sleep needed by each person is usually constant although there is a wide variation among individuals.

How sleep occurs and how it restores the body are not well understood. Scientists originally believed that sleep occurs because the brain lapses into a passive resting state from lack of stimulation. Another theory proposed that sleep occurs when the body generates and accumulates sufficient amounts of a "sleep-inducing substance." However, research now suggests that sleep results when specific changes in brain function occur. By studying brain waves, scientists can define and measure various degrees, levels, and stages of sleep.

Sleep consists of a rhythmic combination of changes in physiological, biochemical, neurophysiological and psychological processes. When the rhythm is disturbed or the individual processes are abnormal during sleep, a variety of sleep-related disorders may result.

Sleep and Breathing Disorders

In 1944, the important observation was made that ventilation (exchange of air between the lung and environment) normally decreases during sleep. Even in "normal" people, breathing patterns during sleep may show a few irregularities. For example, a person might experience an average of seven breathing pauses of up to 10 seconds per night without any associated symptoms or problems. However if the breathing irregularities are accompanied by reduced oxygen supply to tissue (hypoxia) and repeated loss of sleep, these people are at risk of developing more serious problems.

Likely Candidates for Sleep-Related Disorders

Some of the people most likely to have or to develop a sleep-related disorder include:

• adults who fall asleep at inappropriate times and places (e.g., during conversation, lecturing, driving) and who exhibit nighttime snoring
• elderly men and women
• postmenopausal women
• people who are overweight, or have some physical abnormality in the nose, throat, or other parts of the upper airway
• night-shift workers
• people who habitually drink too much alcohol
• blind individuals who tend to develop impaired perception of light and darkness and have disturbed circadian rhythms, the cycles of biologic activities that occur at the same time during each 24 hours
• people with depression and other psychotic disorders.