Apnoea–hypopnoea index
A count of how many times an hour breathing stops or becomes shallow during sleep, used to grade sleep-disordered breathing.
The apnoea-hypopnoea index, usually shortened to AHI, is the standard number used to describe how disrupted someone's breathing is while they sleep. An apnoea is a pause in airflow; a hypopnoea is a partial reduction. The index adds both together and divides by hours of sleep, giving events per hour.
It matters because those repeated interruptions fragment sleep and cause dips in blood oxygen, which is why AHI is used to decide whether sleep-disordered breathing is present and how severe it is. Higher values generally mean more fragmented, less restorative sleep and more daytime sleepiness.
AHI is measured during a sleep study, either in a laboratory or with a home device, using sensors for airflow, breathing effort and blood oxygen. Because it is an objective count rather than a self-report, it is often used alongside questionnaires on sleep quality and daytime sleepiness, which do not always move in step with it.