It is the apnea-hypopnea index, the average number of breathing events per hour across your recording, and it is the headline figure on a sleep study report. An AHI score sorts obstructive sleep apnea into severity bands, and it leaves out several things that matter as much as the number itself.
The bands below are the standard ones. What they do and do not settle comes after.
| Band | Events per hour | What it means |
|---|---|---|
| Normal range | Fewer than five | Not in the diagnostic range for adults |
| Mild | Five to fifteen | Diagnostic, and treatment depends on symptoms |
| Moderate | Fifteen to thirty | Treatment usually recommended |
| Severe | Above thirty | Treatment recommended |
| Supine figure | Reported separately | Can be far higher than the overall number |
| REM figure | Reported separately | Events often cluster in REM sleep |
| Home test figure | Often lower | Calculated over recording time, not sleep time |
| Children | Different criteria | Adult bands do not apply |
Medical Disclaimer
This article is general information about how sleep study results are reported. It is not medical advice and it cannot interpret your result. Only the physician reading your study and your treating clinician can tell you what your numbers mean for you or what treatment is appropriate.
Quick Answer
AHI counts apneas and hypopneas and divides by hours. Adult bands run mild from five to fifteen, moderate from fifteen to thirty, and severe above thirty. The number alone does not settle treatment, because position, sleep stage, oxygen levels and your symptoms all change what it means.
What Counts as an Event
Two event types feed the index, and they are not the same thing.
An apnea is a near-total stop in airflow lasting at least ten seconds. A hypopnea is a partial reduction in airflow, also lasting at least ten seconds, accompanied by a drop in blood oxygen or a brief arousal.
The American Academy of Sleep Medicine publishes the scoring rules that define both, and those definitions are why two labs reading the same night should reach similar numbers.
Hypopneas are the more contested half, because the criteria for how much of a desaturation counts have been revised over time. A report scored under different criteria can produce a different index from the same recording.
That is worth knowing rather than worrying about. It explains why an older result and a newer one are not always directly comparable.
Sleep Time Against Recording Time
This is the single most useful thing to understand about the number, and it is where home and lab results diverge.
An in-lab study records brain activity, so it knows exactly how long you were asleep and divides events by that. The result is events per hour of sleep.
A home test has no brain activity recording, so it cannot tell sleep from lying awake. It divides events by the total time the device was running, which includes every wakeful hour.
Dividing the same number of events by a larger denominator produces a smaller index. So a home figure systematically runs lower than an in-lab figure would for the same person.
Home reports often label this differently, as a respiratory event index, precisely to signal that it is not the same measure. Our guide to telling whether snoring is sleep apnea covers why that distinction shapes what happens after a home result.
Why the Same Number Can Mean Different Things
Two people with an identical index can have very different nights, and the report usually shows why if you read past the headline.
Position is the first reason. Many people have far more events lying on their back than in any other position, so an overall figure averages a severe supine problem with a mild side-sleeping one. Our note on snoring only on your back covers that pattern.
Sleep stage is the second. Events frequently concentrate in REM sleep, where muscle tone is lowest, so a night with little REM produces a lower index than a typical night would. Our guide to understanding sleep cycles covers the staging that reveals this.
Oxygen is the third and arguably the most important. Two people at the same index can have very different desaturation depths, and how far oxygen falls is its own clinical concern.
The National Heart, Lung, and Blood Institute describes sleep apnea’s consequences in terms of interrupted breathing and reduced blood oxygen together rather than in terms of an event count alone.
The Other Indices on Your Report
Respiratory disturbance index
A broader count that can include respiratory effort related arousals, which are events that fragment sleep without meeting apnea or hypopnea criteria.
Oxygen desaturation index
How often blood oxygen fell by a defined amount per hour. It describes the oxygen side rather than the airflow side.
Lowest oxygen saturation
The single lowest reading of the night. A modest index with a very low trough is a different picture from a modest index without one.
Total sleep time and efficiency
How much you slept and what proportion of time in bed that was. Both contextualize everything else on the page.
What to Ask About Your Result
- Ask whether the figure is per hour of sleep or of recording. This single question reframes a home result.
- Ask for the supine figure. A large gap changes what treatment might suit you.
- Ask for the REM figure. Clustering there is common and relevant.
- Ask how low oxygen went. The trough is not visible in the index.
- Ask how much you actually slept. A short night makes every figure less certain.
- Bring your symptoms to the conversation. Sleepiness and function weigh alongside the number.
- Ask what the number means for your options. Mild with heavy symptoms is treated differently from mild without.
Mayo Clinic describes treatment decisions in obstructive sleep apnea as resting on severity together with symptoms and other health conditions rather than on an index in isolation, which is why that last question is the one worth asking most.
Worth reading next: our note on whether CPAP stops snoring and our roundups of CPAP accessories and snore mouthguards cover the two main treatment routes your result may point toward, and our guide to being tired but unable to sleep covers the symptom that most often sits alongside a borderline number.
Frequently Asked Questions
What is an AHI score?
The apnea-hypopnea index: the average number of apneas and hypopneas per hour across a sleep recording. For adults, fewer than five is outside the diagnostic range, five to fifteen is mild, fifteen to thirty is moderate, and above thirty is severe.
What counts as one event?
An apnea is a near-total stop in airflow lasting at least ten seconds. A hypopnea is a partial reduction lasting at least ten seconds with either a drop in blood oxygen or a brief arousal. The American Academy of Sleep Medicine publishes the scoring rules that define both.
Why is my home test number lower?
Because it divides events by the total time the device was recording rather than by the time you were asleep, since a home test does not record brain activity. A larger denominator produces a smaller index, so home figures run systematically lower for the same person.
Is a mild result nothing to worry about?
Not necessarily. Mild with significant daytime sleepiness, or with low oxygen troughs, or alongside other health conditions, is treated differently from mild without any of those. Severity bands sort results; they do not by themselves decide whether treatment is warranted.
What is a supine AHI?
The index calculated for the time you spent on your back. Many people have far more events in that position, so an overall figure averages a severe supine problem with a milder one in other positions. A large gap between the two can change which treatment suits you.
Why does REM sleep matter?
Because muscle tone is at its lowest in REM, events often cluster there. A night with little REM sleep, which is common in an unfamiliar room, can produce a lower overall index than a typical night would. That is one reason the REM figure is reported separately.
What other numbers should I look at?
The oxygen desaturation index and the lowest oxygen saturation of the night, which describe the oxygen side rather than the airflow side, plus total sleep time and sleep efficiency, which tell you how much the whole report rests on. A respiratory disturbance index may also appear.
When should I see a doctor about my result?
Always, since a report is not a treatment plan. See a doctor to go through what the numbers mean for you, including the figures beyond the headline index. Seek prompt care rather than waiting if you have significant daytime sleepiness, witnessed breathing pauses, or you are falling asleep while driving.
Sources
- American Academy of Sleep Medicine. Scoring rules for respiratory events during sleep.
- National Heart, Lung, and Blood Institute. Sleep apnea: symptoms and consequences.
- Mayo Clinic. Obstructive sleep apnea: diagnosis and treatment.
