Diseases affecting the respiratory system are usually present during the day and the night; however, sleep apnea/hypopnea syndrome is a disease manifesting only during sleep. It is defined as a cessation (apnea) or a reduction of breathing (hypopnea) of 10 seconds or more during sleep. Each of these apneas or hypopneas is usually terminated when the person awakens for a few seconds. Typically, patients do not remember these transient awakenings.

There are two general categories of apnea: central and obstructive. These categories are classified according to the presence or the absence of respiratory effort. Central sleep apnea is a result of the absence of nerve impulses from the brain to the respiratory muscles and therefore occurs in absence of respiratory effort. Obstructive sleep apnea is much more common. This condition is caused by repetitive obstruction of the upper airway (pharynx) during sleep. Therefore, respiratory efforts are still present during apnea and usually increase throughout the apnea. The individual makes larger and larger respiratory efforts until they arouse from sleep apnea and reopen their upper airway so breathing can resume. As will be discussed, some overlap exists between obstructive and central apnea in both disease presentation (so-called mixed apnea) and in pathogenesis.