In short
- AHI, the apnea-hypopnea index, is the average number of apneas (breathing pauses) and hypopneas (shallow breathing) per hour of sleep.
- In adults, an AHI under 5 is normal, 5 to under 15 is mild, 15 to under 30 is moderate and 30 or more is severe, according to Cleveland Clinic's summary of the AASM ranges.
- Children are scored on a stricter scale. Cleveland Clinic says an AHI of 1 or more can mean obstructive sleep apnea in a child.
- Under the AASM's recommended rule, a hypopnea is a 30% or greater drop in airflow for at least 10 seconds with a 3% oxygen drop or an arousal. An alternative rule requires a 4% oxygen drop, and the AASM says the choice can make the AHI considerably different.
- Home sleep apnea tests report a respiratory event index (REI), which uses recording time instead of sleep time. The AASM says this can underestimate the true AHI.
- RDI adds respiratory effort-related arousals (RERAs) to the AHI, and ODI counts oxygen drops per hour.
AHI, the apnea-hypopnea index, is the number doctors use to diagnose obstructive sleep apnea and rate how severe it is. It's the average number of times per hour of sleep that your breathing stops or becomes shallow. In adults, under 5 is normal, 5 to under 15 is mild, 15 to under 30 is moderate and 30 or more is severe.
What does AHI stand for in sleep apnea?
Research: strong (what this means)AHI stands for apnea-hypopnea index. Cleveland Clinic defines it as the average number of times you stop breathing (apneas) and have shallow breathing events (hypopneas) per hour of sleep. It's one way to diagnose obstructive sleep apnea and measure how severe it is.
The two kinds of events are counted together because both reduce the air reaching your lungs. Cleveland Clinic explains that after each event, your brain triggers a reflex that wakes you up just enough to breathe again, and most of these arousals are so subtle you don't notice them. That's why people with a high AHI can sleep a full night and still wake up exhausted.
How is an apnea or a hypopnea defined?
An apnea and a hypopnea are defined by specific scoring rules from the American Academy of Sleep Medicine (AASM). The rules matter because they decide what gets counted, and that changes your number.
Under the AASM's 2012 scoring update, an apnea in adults is a drop in airflow of 90% or more from your normal breathing, lasting at least 10 seconds. A hypopnea is a drop of 30% or more for at least 10 seconds, paired with either a 3% or greater fall in blood oxygen or an arousal from sleep.
There's a wrinkle here. The AASM's 2017 diagnostic guideline notes that the scoring manual also allows an alternative hypopnea rule that requires a 4% oxygen drop and ignores arousals. It says that depending on which definition is used, the AHI may be considerably different in the same person. Cleveland Clinic makes a similar point, noting that providers agree on what an apnea is but that the definition of hypopnea involves several factors.
In practice, this means two reports can show different AHIs for the same night. If your number seems surprising, it's reasonable to ask which hypopnea rule was used.
How is AHI calculated?
AHI is calculated by dividing the total number of apneas and hypopneas by the number of hours you slept. Cleveland Clinic gives that formula. So if a sleep study records 120 events over 6 hours of sleep, your AHI is 20.
The data comes from a sleep study. AASM Sleep Education says an in-lab sleep study, also called polysomnography, records your brain waves, heart rate and breathing, along with eye, leg and arm movements and blood oxygen. A sleep technologist scores the study by marking sleep stages and abnormal breathing events, and a sleep doctor reviews it.
The brain wave recording is what lets a lab measure actual sleep time, rather than just time in bed. That detail becomes important with home tests.
What do the AHI severity levels mean?
AHI severity levels sort sleep apnea into four bands for adults. Cleveland Clinic lists the AASM ranges, and the NHS uses nearly the same cutoffs. Children are scored on a stricter scale.
| Severity | Adult AHI (events per hour) | Child AHI (events per hour) |
|---|---|---|
| Normal | Under 5 | Under 1 |
| Mild | 5 to under 15 | 1 to under 5 |
| Moderate | 15 to under 30 | 5 to under 10 |
| Severe | 30 or more | 10 or more |
Cleveland Clinic says a provider will diagnose obstructive sleep apnea in a child with an AHI of 1 or more, and that some providers may use the adult scale for teenagers.
The number isn't the only thing that counts toward a diagnosis. The AASM's adult guideline says sleep apnea is diagnosed with 15 or more events per hour, or 5 or more per hour in someone who has symptoms such as daytime sleepiness, loud snoring or witnessed breathing pauses. So an AHI of 8 means something different in a person who feels fine than in someone falling asleep at their desk. Our guide to sleep apnea warning signs lists the symptoms doctors ask about.
What is the difference between AHI, RDI, REI and ODI?
AHI, RDI, REI and ODI are related measures that count slightly different things, and you may see any of them on a report.
RDI: respiratory disturbance index
RDI adds a third type of event, the respiratory effort-related arousal (RERA). Under the AASM rules, a RERA is a sequence of breaths lasting at least 10 seconds, with increasing breathing effort or a flattened airflow pattern, that leads to an arousal from sleep without meeting the rules for an apnea or hypopnea. The AASM's 2012 scoring update defines RDI as the AHI plus the RERA index. That means RDI is always equal to or higher than AHI. People with symptoms but a low AHI sometimes have a higher RDI, which links to conditions like upper airway resistance syndrome.
REI: respiratory event index
REI is the home-test version of AHI. The AASM's diagnostic guideline describes it as the frequency of apneas and hypopneas derived from a home sleep apnea test. The key difference is the denominator. Most home tests don't record brain waves, so they divide by recording time instead of actual sleep time.
ODI: oxygen desaturation index
ODI counts how many times per hour your blood oxygen drops by a set amount. The AASM's 2012 update defines it using 3% drops. Some studies use 4%. For example, the STAR trial of upper airway stimulation defined ODI as drops of 4 percentage points or more. ODI only looks at oxygen, so it can miss events that cause arousals without much oxygen change.
Is a home sleep test AHI as accurate as a lab test?
A home sleep test is accurate enough to diagnose many people, but it tends to read lower than a lab study. The AASM's 2017 guideline recommends that either polysomnography or a technically adequate home test can be used to diagnose uncomplicated adults with signs of moderate to severe sleep apnea.
The same guideline explains why home tests can underestimate the true AHI. They use recording time rather than sleep time, so time spent lying awake dilutes the count. Devices with conventional sensors also can't detect hypopneas that cause only brain arousals. Sensors can slip off or record poor signals too.
That's why a low result isn't always the final word. AASM Sleep Education says a negative home sleep apnea test is not sophisticated enough to rule out sleep apnea, and the guideline recommends an in-lab study if a single home test is negative, inconclusive or technically inadequate. Sleep Education also says home tests aren't suitable for people with certain lung, heart or neuromuscular conditions. Our guide to the home sleep apnea test walks through the process.
What does the AHI on a CPAP machine mean?
The AHI shown on a CPAP machine is an estimate of how many events are still getting through while you use it. Cleveland Clinic says the machine detects changes in your breathing to estimate your AHI, that it isn't an exact measurement, and that it gives your provider useful information on how well treatment is working.
The usual goal for adults on CPAP is an AHI below 5, according to Cleveland Clinic, though you and your provider decide the target. If your machine number creeps up, that's a reason to check in rather than change settings yourself. Our comparison of mouth tape and CPAP explains why tape isn't a substitute for treatment.
What are the limits of AHI?
AHI is the standard measure, but it doesn't tell the whole story. Cleveland Clinic notes that it only counts apneas and hypopneas and says nothing more about how severe each event is. A 10-second pause and a 60-second pause with a deep oxygen drop count the same.
That's why your sleep doctor looks at more than one number. Cleveland Clinic says your care team will use AHI alongside other results, such as your sleep position and AHI during each sleep stage. Position matters a lot for some people, as our guide to positional sleep apnea explains.
Can a single test miss sleep apnea?
A single test can miss sleep apnea, even in a lab. The AASM's 2017 guideline suggests that when a first in-lab study is negative and a doctor still suspects sleep apnea, a second study be considered. It makes the same point more strongly for home tests, recommending an in-lab study after a negative, inconclusive or technically inadequate home test.
AASM Sleep Education adds that many people don't sleep as well in the lab as at home, though it says this typically doesn't affect the results, and that you usually don't need a full eight hours of sleep for a diagnosis. If your symptoms don't match your number, say so. That mismatch is exactly what a repeat test is for.
What should you do with your number?
Your AHI is a starting point for a conversation, not a verdict on its own. If your result is mild, ask whether lifestyle changes or positional therapy could be enough. If it's moderate or severe, ask about the treatment options that fit your results. And if your AHI is normal but you still feel unrefreshed, ask whether RDI was measured or whether a lab study makes sense.
It also helps to know that the number can change. Weight, alcohol and sleep position all move it, which our article on whether sleep apnea can go away covers in detail.
This article is general information, not medical advice. Your sleep doctor can explain what your own report shows.
Read next
Common questions
What does AHI mean in sleep apnea?
AHI stands for apnea-hypopnea index. It's the average number of times per hour of sleep that your breathing stops (apnea) or becomes shallow (hypopnea). Doctors use it to diagnose obstructive sleep apnea and rate how severe it is.
What is a normal AHI?
For adults, an AHI under 5 events per hour is considered normal, according to Cleveland Clinic. For children, a normal score is under 1. A score in the normal range on one test doesn't rule out other sleep problems, so symptoms still matter.
What AHI is considered severe sleep apnea?
An AHI of 30 or more events per hour is considered severe in adults. Moderate is 15 to under 30, and mild is 5 to under 15. These are the AASM ranges as summarized by Cleveland Clinic, and the NHS uses nearly identical cutoffs.
What is a good AHI on CPAP?
Cleveland Clinic says the goal for adults using CPAP is usually an AHI below 5. The number on your machine is an estimate based on airflow, not a full sleep study, so it's best reviewed with your doctor alongside how you feel.
What is the difference between AHI and RDI?
RDI, the respiratory disturbance index, includes everything in the AHI plus respiratory effort-related arousals (RERAs), which are smaller breathing disruptions that wake the brain briefly. The AASM's 2012 scoring update defines RDI as AHI plus the RERA index, so RDI is always the same as or higher than AHI.
Is a home sleep test AHI accurate?
It's useful, but it can read low. The AASM guideline says home tests use recording time rather than sleep time and can't detect hypopneas that cause only brain arousals, both of which can underestimate the true AHI. A negative or unclear home test should be followed by an in-lab sleep study.
Sources
- Apnea-hypopnea index (AHI)
- Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events
- Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline
- Clinical guideline for the evaluation, management and long-term care of obstructive sleep apnea in adults
- Sleep apnoea
- Home sleep apnea test
- Sleep study
- Upper-airway stimulation for obstructive sleep apnea
About this article
- Research
- 8 published sources, listed above.
- Evidence
- Strong. How we rate it
- Last checked
- October 14, 2026
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* This content is for informational and educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure or prevent any disease or health condition. Talk to a qualified healthcare provider about your health.