Sleep apnea is a sleep disorder

Sleep disorder

A sleep disorder, or somnipathy, is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental and emotional functioning...

 characterized by abnormal pauses in breathing or instances of abnormally low breathing, during sleep

Sleep

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

. Each pause in breathing, called an apnea

Apnea

Apnea, apnoea, or apnœa is a term for suspension of external breathing. During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged...

, can last from a few seconds to minutes, and may occur 5 to 30 times or more an hour. Similarly, each abnormally low breathing event is called a hypopnea. Sleep apnea is diagnosed with an overnight sleep test called a polysomnogram, or "sleep study".

There are three forms of sleep apnea: central

Central sleep apnea

Central sleep apnea or central sleep apnea syndrome is a sleep-related disorder in which the effort to breathe is diminished or absent, typically for 10 to 30 seconds, either intermittently or in cycles and is usually associated with a reduction in blood oxygen saturation...

 (CSA), obstructive

Obstructive sleep apnea

Obstructive sleep apnea or obstructive sleep apnea syndrome is the most common type of sleep apnea and is caused by obstruction of the upper airway. It is characterized by repetitive pauses in breathing during sleep, despite the effort to breathe, and is usually associated with a reduction in...

 (OSA), and complex or mixed sleep apnea (i.e., a combination of central and obstructive) constituting 0.4%, 84% and 15% of cases respectively.