How this instrument works
The Apnea-Hypopnea Index adds two counts from an overnight sleep study, apneas and hypopneas, and divides by the hours of total sleep time recorded, giving events per hour. An apnea is a complete or near-complete pause in airflow; a hypopnea is a partial reduction, shallow enough to disrupt sleep and lower blood oxygen without stopping breathing outright. Both count toward the index, because both fragment sleep and interrupt oxygenation even though only one of them is a full stop.
The index is calculated against total sleep time, not time spent in bed. A formal study measures this with EEG, tracking brain-wave patterns to mark exactly when a person is actually asleep versus lying awake, because events per hour only means something if the hours behind it are hours of real sleep. Someone who spends eight hours in bed but sleeps only six will get a materially different AHI depending on which of those two numbers the count is divided by.
The American Academy of Sleep Medicine's 1999 task force report set the severity bands still used today: under 5 events per hour is normal, 5 to 15 is mild, 15 to 30 is moderate, and above 30 is severe. These thresholds come from population data linking event frequency to daytime symptoms and downstream health risk, not from one clean biological cutoff, which is why a result sitting right at a boundary, like 5.0, gets read in context rather than as a hard verdict.
- Enter Apneas — the count of complete breathing pauses recorded during the study.
- Enter Hypopneas — the count of partial airflow reductions recorded separately.
- Enter Total sleep time in hours — the EEG-measured hours actually asleep, not time in bed.
- Read the Apnea-Hypopnea Index in events per hour, then compare it against the severity bands in the FAQ.
Worked example — three sleep studies
20 apneas and 15 hypopneas over 7 hours of recorded sleep: (20 + 15) ÷ 7 = 5.0 events per hour, landing exactly on the boundary between normal and mild. A second study with 60 apneas and 40 hypopneas over 6.5 hours works out to (60 + 40) ÷ 6.5 = 15.4 events per hour, into the moderate band.
A third case: 150 apneas and 80 hypopneas over 7 hours of sleep gives (150 + 80) ÷ 7 = 32.9 events per hour, above 30, the severe threshold. Notice the third study has fewer sleep hours than double the first but more than six times the combined event count, exactly what a per-hour rate is built to make comparable across nights of different length.
Questions
What is the difference between an apnea and a hypopnea?
An apnea is a near-total stop in airflow, breathing effectively halts for at least ten seconds. A hypopnea is a partial reduction, shallower breathing that still drops airflow enough to disturb sleep and lower blood oxygen without stopping it completely. Both events fragment sleep architecture, which is why the index counts them together rather than scoring only the more severe apneas.
Why is AHI calculated against total sleep time instead of time in bed?
Because events per hour is only a meaningful rate if the hours behind it are hours of actual sleep. Someone might spend eight hours in bed but sleep only six of them; dividing by eight would understate how often the events are really occurring during sleep. A sleep study measures total sleep time with EEG specifically so this denominator is accurate rather than merely convenient.
What do the AHI severity bands mean?
Under 5 events per hour is normal, 5 to 15 is mild, 15 to 30 is moderate, and above 30 is severe, per the American Academy of Sleep Medicine's 1999 task force criteria still used in clinical practice. These bands correlate with daytime symptoms and long-term cardiovascular risk in population studies, but any single person's number is read alongside symptoms, not in isolation.
Is an AHI of exactly 5.0 normal or mild?
By the standard convention, under 5 is normal and 5 and above is mild, so 5.0 itself falls just inside the mild category. It is a boundary case in every sense: the underlying event counts and sleep-time measurement both carry some night-to-night variability, so a result this close to the cutoff is often followed up with a repeat study or clinical correlation rather than treated as decisive on its own.
Can AHI be measured at home instead of in a sleep lab?
Home sleep apnea tests exist and estimate a similar index, but most do not measure brain waves directly, so they typically divide by total recording time rather than true EEG-confirmed sleep time. The result from those devices is often called a Respiratory Event Index rather than AHI for this reason, and it tends to run a little lower than a lab AHI would for the same person.
Does a high AHI always mean obstructive sleep apnea?
Usually, but not automatically. The index measures event frequency without distinguishing cause, so a sleep physician also looks at the pattern of events, oxygen desaturation, and other study data to separate obstructive apnea from central apnea, where the brain briefly stops signaling the breathing muscles, or other causes. AHI is one number in a fuller diagnostic picture, not a diagnosis by itself.
References
- American Academy of Sleep Medicine Task Force, 1999, Sleep — syndrome definitions (PubMed)
- Sleep Foundation — Apnea-Hypopnea Index (AHI) and Sleep Apnea
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.