Sleep apnea severity is measured by how often the person stops breathing during sleep, and how long each apnea lasts. The most common measure of sleep apnea severity is the Apnea-Hypopnea Index (AHI).
The Apnea-Hypopnea Index (AHI) is calculated by dividing the number of apneas and hypopneas by the total number of hours of sleep. An AHI of 5 or more is considered severe sleep apnea.