apnea–hypopnea index

The apnea–hypopnea index, or AHI, is the single number that sums up a night of disordered breathing into one rate: how many times per hour, on average, breathing either stopped (apneas) or shrank meaningfully (hypopneas). Think of it as the hourly tally of breathing interruptions while asleep. An AHI of 30 means thirty events every hour — roughly one every two minutes.

It is calculated by counting all qualifying apneas and hypopneas across the sleep study and dividing by the hours of actual sleep recorded. A common severity scheme labels fewer than 5 events per hour as normal, 5 to under 15 as mild, 15 to under 30 as moderate, and 30 or more as severe. Because it depends on time asleep, accurate brain-wave staging makes the number more reliable than counting events over time merely in bed.

The AHI is the workhorse metric for diagnosing and grading sleep apnea and for deciding who needs treatment, but it is a summary, not the whole story. It does not capture how deep the oxygen dips go, how long they last, how fragmented the sleep is, or how sleepy the person feels. Two people with the same AHI can differ greatly in symptoms and cardiovascular risk.

Be aware of important caveats: the AHI rises in lighter sleep stages and in the back-lying position and tends to be worse in REM sleep, so a single night can under- or over-represent the true burden. Home tests that estimate events over total recording time (rather than actual sleep) often report a related but different figure, the respiratory event index.