Sleep apnea is a sleep disorder in which pauses in breathing or periods of shallow breathing during sleep occur more often than normal. It occurs most commonly in males, and in the obese. It affects approximately 3 to 7 percent of adults. It sometimes involves upper airway obstruction and is characterized by cessation of breathing and a marked fall in blood oxygen levels, thus arousing an affected individual from sleep.

During sleep, body metabolism is reduced, but there is an even greater decline in ventilation so that the partial pressure of carbon dioxide in arterial blood rises slightly and arterial partial pressure of oxygen falls. Because arousal is often associated with the termination of episodes of obstruction, sleep is of poor quality, and complaints of excessive daytime drowsiness are common. Snoring and disturbed behaviour during sleep may also occur.

In some persons with sleep apnea, portions of the larynx and pharynx may be narrowed by fat deposits or enlarged tonsils and adenoids, which increase the likelihood of obstruction. Others, however, have normal upper airway anatomy, and obstruction may occur because of discordant activity of upper airway and chest wall muscles. Many of the upper airway muscles, like the tongue and laryngeal adductors, undergo phasic changes in their electrical activity synchronous with respiration, and the reduced activity of these muscles during sleep may lead to upper airway closure.

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