Sleep apnea or sleep apnoea is a sleep disorder characterized by pauses in breathing during sleep. These episodes, called apneas (literally, "without breath"), each last long enough so one or more breaths are missed, and occur repeatedly throughout sleep. The standard definition of any apneic event includes a minimum 10 second interval between breaths, with either a neurological arousal (3-second or greater shift in EEG frequency, measured at C3, C4, O1, or O2), or a blood oxygen desaturation of 3-4 percent or greater, or both arousal and desaturation. Sleep apnea is diagnosed with an overnight sleep test called polysomnogram.
Clinically significant levels of sleep apnea are defined as 5 events of any type or greater per hour of sleep time (from the polysomnogram). There are two distinct forms of sleep apnea: Central and Obstructive. Breathing is interrupted by the lack of effort in Central Sleep Apnea; in Obstructive Sleep Apnea, breathing is interrupted by a physical block to airflow despite effort. In Mixed Sleep Apnea, there is a transition from central to obstructive features during the events themselves.
Regardless of type, the individual with sleep apnea is rarely aware of having difficulty breathing, even upon awakening. Sleep apnea is recognized as a problem by others witnessing the individual during episodes or is suspected because of its effects on the body (sequelae). Symptoms may be present for years, 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. The definitive diagnosis of sleep apnea is made by polysomnography.