AHI, the apnea-hypopnea index, is the number of apneas (breathing stops) and hypopneas (partial reductions) per hour of sleep. On a sleep study it is the number the report is built on. On your CPAP it is the machine's estimate of how many events got through on therapy.
Does it matter? Yes as a trend, less as a single night, and it never tells the whole story.
Medical disclaimer: AirwayLab is a data-visualization tool, not a medical device. Nothing in this article constitutes medical advice, diagnosis, or treatment. Always discuss your therapy data and any concerns with your sleep physician or qualified clinician.
What counts as an event
AHI is calculated as apneas plus hypopneas, divided by hours of sleep, the count of qualifying events per hour of the night [S1]. Two event types feed that count, each with its own definition.
An apnea (a breathing stop) is an airflow drop of at least 90% lasting at least 10 seconds [S1].
A hypopnea (a partial airflow reduction) is an airflow drop of at least 30% lasting at least 10 seconds, with either a 3% (or 4%, depending on the scoring rule used) oxygen desaturation or an arousal [S1, S3].
The severity bands
A sleep study classifies AHI into four severity bands, from normal to severe [S1]:
| AHI (events/hour) | Band |
|---|---|
| Under 5 | Normal |
| 5 to under 15 | Mild |
| 15 to under 30 | Moderate |
| 30 and over | Severe |
These are severity bands from a sleep study, not treatment targets for your nightly CPAP number.
AHI on CPAP is an estimate
A CPAP machine's AHI is a device estimate from flow alone: it has no EEG and no oximeter, so it divides by hours of use, not hours of sleep, and it cannot see arousals or desaturations, which is why it can differ from the lab number.
PAP therapy commonly aims for a residual AHI (your CPAP's own count, on therapy) under 5, but the guideline stresses symptoms and adherence, not the number alone [S2].
AirwayLab shows AHI per night next to leak and flow limitation, and opens the waveform behind each flagged breath, so you can see what the number is made of.
Check your own nights
Upload your SD card and see AHI, leak and flow limitation per night, in plain language. Free.
What AHI does not count
Flow limitation (a partial narrowing of the airway that does not meet the hypopnea threshold) is not counted in AHI.
RERAs (arousals from effortful breathing that never drops far enough to score as an event) are not counted in AHI either.
When the number and the feeling disagree
- Leak (air escaping around the mask seal) can mask or distort what the machine scores, see how leak rate is measured and bring the data to your provider.
- Some events the machine cannot classify cleanly, see what a good AHI on CPAP looks like and bring the data to your provider.
- Sleep fragmentation without countable events can still leave you tired, see why a normal AHI does not always mean rested sleep and bring the data to your provider.
Related reading
See the AHI glossary entry for a quick reference, or read how AirwayLab measures your data for the full methodology.
Check your own nights
Upload your SD card and see AHI, leak and flow limitation per night, in plain language. Free.
Sources
- [S1] American Academy of Sleep Medicine. The AASM Manual for the Scoring of Sleep and Associated Events, Version 3 (2023). Grade: clinical scoring standard.
- [S2] Patil SP et al. Treatment of Adult Obstructive Sleep Apnea with Positive Airway Pressure: An American Academy of Sleep Medicine Clinical Practice Guideline. J Clin Sleep Med 2019;15(2):335-343. Grade: clinical practice guideline.
- [S3] Berry RB et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual. J Clin Sleep Med 2012;8(5):597-619. Grade: clinical scoring standard.
AirwayLab is a data-visualization tool, not a medical device. The metrics described in this article are data recorded by your CPAP device. Nothing on this page constitutes medical advice, diagnosis, or treatment. Always discuss your therapy data and any concerns with your sleep physician or qualified clinician.