Sleep apnea, sleep apnoea or sleep apnœa 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, or a blood oxygen desaturation of 3-4 percent or greater, or both arousal and desaturation.

Clinically significant levels of sleep apnea are defined as 5 or more events of any type per hour of sleep time. There are three distinct forms of sleep apnea: Central, Obstructive and Complex (complex is a combination of central and obstructive) with 0.4%, 84% and 15% making up their respective percentages of cases. 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. 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.

Obstructive is more frequent than central sleep apnea; it is a common condition in many parts of the world. If studied carefully in a sleep lab by polysomnography, approximately 1 in 5 American adults has at least mild obstructive sleep apnea. Since the muscle tone of the body ordinarily relaxes during sleep, and since, at the level of the throat, the human airway is composed of walls of soft tissue, which can collapse, it is easy to understand why breathing can be obstructed during sleep. Although many individuals experience episodes of obstructive sleep apnea at some point in life, a much smaller percentage of people are afflicted with chronic severe obstructive sleep apnea.

In pure central sleep apnea or Cheyne-Stokes respiration, the brain's respiratory control centers are imbalanced during sleep. Blood levels of carbon dioxide, and the neurological feedback mechanism that monitors it do not react quickly enough to maintain an even respiratory rate, with the entire system cycling between apnea and hyperpnea, even during wakefulness. The sleeper stops breathing, and then starts again. There is no effort made to breathe during the pause in breathing: there are no chest movements and no struggling. After the episode of apnea, breathing may be faster (hyperpnea) for a period of time, a compensatory mechanism to blow off retained waste gases and absorb more oxygen.