AHI is the apnea-hypopnea index: the average number of breathing events per hour of sleep, counting both the full stops and the partial collapses. It is the number your machine shows you in the morning, and it is the closest thing you have to a nightly answer about whether the therapy is doing its job.
What the ranges mean
In diagnosis, the conventional bands are these — and they describe untreated sleep, which is why the number you were given at diagnosis is usually so much larger than the one your machine reports now.
- Under 5 — normal.
- 5 to 15 — mild.
- 15 to 30 — moderate.
- 30 and above — severe.
What counts as good on CPAP
The entire point of the pressure is to splint the airway open so the treated figure lands back under 5. Many people settle somewhere around 1 or 2, and lower is generally better.
One high night is not a crisis. A cold, a late glass of wine, or a night spent on your back will each move the number, and reading a single reading as a verdict is the fastest way to make yourself miserable about a therapy that is working. What matters is the trend.
Why a creeping number is your early warning
A figure that climbs steadily over a week or two is almost always something with a fix behind it: a mask leaking pressure away, a cushion that has hardened with wear, a change in weight, or a prescribed setting that has drifted out of step with you.
Caught early, that is a five-minute repair. Caught late, it is a month of sleeping badly while assuming this is simply what CPAP feels like — and a month of that is how people talk themselves into stopping.
Nightshift charts the figure night by night, separates the one-off from the real drift, and flags a climb with the likeliest cause. It does not diagnose, and it does not change your pressure — your sleep clinician sets your target and your settings.