Sleep apnea is a type of sleep disorder characterized by pauses in breathing or instances of shallow or infrequent breathing during sleep. Each pause in breathing, called "an apnea", can last from at least ten seconds to several minutes, and may occur 5 to 30 times or more an hour.  51 , each abnormally shallow breathing event is called a "hypopnea". Sleep apnea is classified as a dyssomnia, meaning abnormal behavior or psychological events occur during sleep. When breathing is paused, carbon dioxide is building up in the bloodstream. Chemoreceptors in the blood stream note the high carbon dioxide levels. The brain is  52  to wake the person sleeping and to breathe in air. Breathing normally will restore oxygen levels and the person will fall asleep again. Sleep apnea is often  53  with an overnight sleep test called a "polysomnogram", or "sleep study".

There are three forms of sleep apnea, central (CSA), obstructive (OSA), and complex or mixed sleep apnea (i.e., a combination of central and obstructive)  54  0.4%, 84% and 15% of cases  55 . In CSA, breathing is interrupted by a lack of respiratory effort; in OSA, breathing is interrupted by a physical block to airflow despite the respiratory effort, and  56  is common. According to the National Institutes of Health, 12 million Americans have OSA. There are more cases of sleep apnea still because people either do not report the  57  or do not know they have sleep apnea.

Regardless of type, an individual with sleep apnea is rarely aware of having difficulty  58 , even upon awakening. Sleep apnea is recognized as a problem by others witnessing the individual during episodes or is  59  because of its effects on the body (sequelae). Symptoms  60  for years or even decades without identification, during which time the sufferer may become conditioned to the daytime sleepiness and fatigue associated with significant levels of sleep disturbance.