Sleep apnea is measured by counting how many times your breathing stops or becomes shallow each hour while you sleep. That number, called the apnea-hypopnea index (AHI), is the single most important metric in diagnosing and grading the condition. An AHI under 5 is normal. Five or above, combined with symptoms like daytime sleepiness or loud snoring, means you have obstructive sleep apnea.
The AHI Score and What It Means
The AHI is calculated by adding up every apnea (a complete pause in breathing) and every hypopnea (a partial reduction in airflow) that occurs during sleep, then dividing by the total hours slept. If you had 120 events over 8 hours, your AHI would be 15.
Harvard Medical School’s Division of Sleep Medicine classifies severity using these thresholds:
- Normal: AHI below 5 events per hour
- Mild: AHI of 5 to 14
- Moderate: AHI of 15 to 29
- Severe: AHI of 30 or higher
You can also receive a diagnosis without any symptoms at all if your AHI reaches 15 or above. Below that, a diagnosis requires the presence of at least one characteristic symptom: unrefreshing sleep, excessive daytime sleepiness, witnessed breathing pauses, waking up gasping or choking, or loud habitual snoring.
In-Lab Sleep Studies
Polysomnography, the overnight sleep study conducted in a sleep lab, is the gold standard for diagnosing obstructive sleep apnea. The American Academy of Sleep Medicine recommends it as the primary diagnostic test for adults when a comprehensive sleep evaluation raises concern.
During the study, a technician attaches sensors that monitor brain waves, eye movements, muscle activity, heart rhythm, airflow through your nose and mouth, chest and abdominal movement, and blood oxygen levels. Together, these signals let a sleep specialist determine not only how many breathing disruptions you have, but also whether each event is obstructive (caused by a physical blockage in the airway) or central (caused by the brain failing to signal the muscles to breathe). The brain wave data also tracks your sleep stages, which matters because apnea events often cluster in certain stages, particularly during REM sleep when muscle tone drops the most.
You typically arrive at the lab in the evening, sleep overnight with the sensors in place, and leave the next morning. Results are scored by a trained technologist and interpreted by a sleep physician, usually within one to two weeks.
Home Sleep Apnea Tests
Home sleep apnea tests (HSATs) are a simpler, more convenient alternative for people with a high likelihood of moderate to severe obstructive sleep apnea and no other significant sleep disorders. They use fewer sensors and you wear them in your own bed.
A standard home test (classified as a Type 3 device) records four to six signals: at least two measures of respiratory effort, blood oxygen saturation, and heart rate or pulse. Some stripped-down versions (Type 4) track just one or two signals, typically oxygen levels or airflow alone. You pick up the device from your doctor’s office or receive it by mail, wear it for one or two nights, and return it for analysis.
The trade-off is accuracy. Home tests don’t monitor brain activity, so they can’t tell how long you actually slept. Instead of dividing breathing events by hours of sleep, they divide by hours of recording time. If you spent an hour lying awake, your score gets diluted, potentially underestimating your true AHI. For this reason, a negative or borderline home test in someone with strong symptoms often leads to a follow-up in-lab study.
Oxygen-Based Measurements
Beyond the AHI, oxygen levels during sleep offer an independent window into how much damage each breathing pause does. Two metrics matter most.
The oxygen desaturation index (ODI) counts how many times per hour your blood oxygen drops by 3% or more from its recent baseline, with each dip lasting at least 10 seconds. A high ODI tells you that breathing events are actually starving your body of oxygen, not just disrupting airflow. Because repeated oxygen drops drive many of the cardiovascular complications of sleep apnea, the ODI is increasingly used alongside the AHI to gauge real-world severity.
The nadir oxygen saturation is simply the lowest point your blood oxygen reaches all night. Research published in Frontiers in Cardiovascular Medicine found that this single number was one of the strongest factors correlated with acute heart attack risk in people with sleep apnea, outperforming the AHI and ODI. A nadir below 80% is a red flag that warrants close attention, though even dips into the mid-80s over hundreds of events per night add up.
The RDI: A Broader Measure
Some sleep reports include a respiratory disturbance index (RDI) instead of, or alongside, the AHI. The RDI captures everything the AHI does plus a category of subtler events called respiratory effort-related arousals. These are moments when your airway narrows enough to disturb your sleep and wake your brain briefly, but not enough to meet the technical definition of an apnea or hypopnea. Because the RDI casts a wider net, it tends to produce a higher number than the AHI from the same night of sleep. The formal diagnostic criteria from the International Classification of Sleep Disorders use the RDI as the threshold for diagnosis: 5 or more events per hour with symptoms, or 15 or more without.
Consumer Wearables
Apple received FDA clearance in September 2024 for a Sleep Apnea Notification Feature on its smartwatch, classified as an over-the-counter device to assess the risk of sleep apnea. Samsung has a similar feature. These devices use motion and oxygen sensors to flag patterns consistent with moderate to severe apnea, but they are screening tools, not diagnostic instruments. A notification from a wearable is a reason to pursue formal testing, not a diagnosis on its own. The sensors are far less precise than clinical-grade equipment, and they cannot distinguish obstructive apnea from central apnea or other sleep disorders that fragment breathing.
Children Have Different Thresholds
If you’re looking into sleep apnea measurement for a child, the numbers shift dramatically. In children 13 and younger, an AHI of just 1 event per hour is considered abnormal. The same AHI that would be labeled “normal” in an adult could indicate meaningful obstruction in a child, because a developing brain and body are more vulnerable to even small amounts of fragmented sleep and intermittent oxygen drops. Polysomnography is the recommended diagnostic test for pediatric cases, and home tests are generally not validated for children.