Hypopneas occur when a person takes shallow breaths for ten seconds or longer during sleep. This happens when the airway is partially blocked and results in airflows of 30% or less.
Apneas and hypopneas are measured using the apnea-hypopnea index (AHI) which measures the number of apneas and hypopneas that occur per hour. The greater the number, the worse the sleep breathing disorder.