Apnea: An apnea is a period of time during which breathing stops or is markedly reduced. There are two types of apneas, the more common obstructive sleep apnea and the less common central sleep apnea.

Apneas usually occur during sleep , and when they do occur, sleep is usually disrupted. Sometimes the person wakes up completely, but sometimes the person comes out of a deep level of sleep and into a more shallow level of sleep. Apneas are usually measured during sleep (preferably in all stages of sleep). An estimate of the severity of apnea is calculated by dividing the number of apneas by the number of hours of sleep, giving an apnea index (AI). The greater the apnea index, the more severe the apnea.