Hypopnea vs Apnea: What the Difference Means for Your AHI

Quick answer: An apnea is a near-complete stop in airflow for at least ten seconds. A hypopnea is a partial reduction in airflow that lasts at least ten seconds and is usually accompanied by an oxygen drop or an arousal. Both count toward the apnea-hypopnea index (AHI), but they are scored differently and can have different effects on oxygen levels and sleep quality.

What is an apnea?

An apnea is a breathing pause during sleep. In adults, the standard definition is a reduction in airflow of at least 90 percent that lasts at least ten seconds. In plain terms, the airway is almost completely closed or the drive to breathe has dropped out.

Apneas are classified by effort. An obstructive apnea occurs when the airway is blocked but the chest and abdomen still move, showing that the body is trying to breathe. A central apnea occurs when there is no airflow and no respiratory effort, because the brain's breathing signal has temporarily paused. A mixed apnea starts as central and becomes obstructive.

Obstructive apneas are the most common type in people with obstructive sleep apnea (OSA). Central apneas are more common in heart failure, after a stroke, with certain medicines, and at high altitude.

What is a hypopnea?

A hypopnea is a partial reduction in airflow, not a complete stop. The standard definition is a reduction of at least 30 percent that lasts at least ten seconds, with either an oxygen desaturation of at least 3 percent or an arousal from sleep.

Some laboratories and payers use a 4 percent desaturation threshold instead of 3 percent. This can change the number of events that are scored and, in turn, the AHI. It is one reason two reports may not look identical even when the underlying breathing is similar.

Most hypopneas in OSA are obstructive, but hypopneas can also occur in central sleep apnea. The technologist looks at airflow, effort, oxygen saturation, and arousal to classify the event.

How AHI is calculated

The AHI is the number of apneas plus hypopneas per hour of sleep. If a report lists 40 apneas and 60 hypopneas over seven hours of sleep, the AHI is about 14 events per hour. The formula matters because both event types count.

Severity is usually grouped as:

• Mild: AHI 5 to 15 events per hour.
• Moderate: AHI 15 to 30 events per hour.
• Severe: AHI above 30 events per hour.

These cutoffs are useful for research and treatment guidelines, but they are not a complete description of a person's experience. Someone with a lower AHI and severe symptoms may need treatment, while someone with a higher AHI and no symptoms may still need a careful discussion.

The guide to sleep study results explained covers AHI, oxygen saturation, and other report terms in more detail.

How scoring works in practice

A sleep study is not a single continuous recording that a computer turns into a number. A technologist reviews the tracing in 30-second segments, looking at airflow, breathing effort, oxygen saturation, and brain-wave arousals. The technologist decides whether each event meets the apnea or hypopnea criteria and whether it is obstructive, central, or mixed.

Artifacts can complicate the picture. A mouth leak, a displaced sensor, or a poor oxygen signal can make an event look different from what it was. Position and sleep stage also matter: events that occur only on the back or only during REM sleep may be missed if those periods are short.

Scoring is a human process, and some variability between technologists is expected. If a report does not match your symptoms, ask whether the raw data can be reviewed. A repeat study may be useful when the first test was technically inadequate or when the clinical picture has changed. The guide to sleep studies explains the testing process from the patient's side. Ask for the event breakdown if it is not shown; the total AHI alone can hide the pattern.

Why the difference matters

Apneas and hypopneas can both fragment sleep and lower oxygen, but the degree varies. A long obstructive apnea may cause a larger oxygen drop and a stronger arousal than a brief hypopnea. A hypopnea with repeated arousals can still leave a person exhausted even if the oxygen nadir looks acceptable.

The pattern also matters. Events that cluster in REM sleep or in the supine position may have a disproportionate effect. Some people have mostly hypopneas and are still very sleepy. Others have mostly apneas but few symptoms. A clinician should look at the whole picture, not just the total AHI.

This is also why the term "mild" can be misleading. Mild OSA by AHI can still cause significant daytime sleepiness, high blood pressure, or driving risk. The guide to mild obstructive sleep apnea explains how symptoms and risk factors influence treatment.

Obstructive vs central events

The apnea-hypopnea distinction is separate from the obstructive-central distinction. An event can be an obstructive apnea, a central apnea, an obstructive hypopnea, or a central hypopnea. The AHI combines them, but the treatment can be very different.

Obstructive events usually respond to CPAP, oral appliances, positional therapy, or surgery. Central events may respond to treating the underlying cause, changing medicines, or using specialized positive airway pressure modes. The guide to central versus obstructive sleep apnea explains the differences.

A report that only gives the total AHI is less useful than one that breaks out event types, oxygen levels, and position. Ask for the details if they are not in the summary.

What else your sleep study reports

AHI is not the only number that matters. A useful report may include:

• Oxygen nadir: the lowest oxygen saturation during sleep.
• Time below 90 percent: how long oxygen stayed low.
• Arousal index: how often sleep was briefly interrupted.
• Respiratory effort-related arousals (RERAs): events that do not meet the apnea or hypopnea criteria but still disrupt sleep.
• Positional data: whether events are worse on the back.
• REM data: whether events cluster in REM sleep.
• Sleep stages and efficiency: how much time was spent asleep and in each stage.

Some reports include a respiratory disturbance index (RDI), which adds RERAs to the AHI. Not every laboratory reports RDI, and insurance rules may not use it. Ask which index the clinician is using and why.

How hypopneas and apneas are treated

Treatment is based on the diagnosis, severity, symptoms, and health risks. For obstructive sleep apnea, options include positive airway pressure (CPAP), oral appliances, positional therapy, weight management, and surgery in selected cases. The presence of hypopneas rather than apneas does not automatically change the first-line treatment, but it can affect how the response is measured.

Follow-up matters. A device that reduces apneas but leaves many hypopneas may not fully resolve symptoms. A repeat study or device download can show whether the AHI and oxygen levels have improved. The goal is not a perfect number on a report; it is better sleep, safer breathing, and reduced health risk.

If you are starting treatment, the guide to CPAP getting started covers fitting and troubleshooting. If you are considering an oral appliance, the guide to oral appliance therapy versus CPAP explains the tradeoffs.

To discuss your report with a clinician, you can find a sleep specialist near you. The directory lists clinics and services; it does not interpret individual studies.

Frequently asked questions

Is a hypopnea as serious as an apnea?

It depends on the length, oxygen drop, arousal, and how often it happens. Hypopneas can cause significant sleep fragmentation and daytime sleepiness. A clinician should interpret the pattern, not just the label.

Does AHI include hypopneas?

Yes. The AHI is the number of apneas plus hypopneas per hour of sleep. Some reports also include a separate index for respiratory effort-related arousals.

Why do some reports use 3 percent and others 4 percent?

The AASM scoring rules allow a 3 percent desaturation or arousal for hypopneas, while some payers and laboratories use a 4 percent threshold. The choice can change the AHI and insurance eligibility.

Can you have sleep apnea with only hypopneas?

Yes. A person can meet the criteria for OSA with hypopneas alone. The diagnosis depends on the AHI, symptoms, and other findings.

What is a RERA?

A respiratory effort-related arousal is a sequence of increasing breathing effort that ends in an arousal without meeting the full apnea or hypopnea criteria. RERAs can contribute to sleep fragmentation and are included in the RDI.

Which number should I focus on?

AHI is important, but it is not the only measure. Ask about oxygen levels, arousals, symptoms, and how you feel during the day. The treatment plan should reflect the whole picture.

How we built this guide

This guide draws on public information from the American Academy of Sleep Medicine's patient education resources and the National Heart, Lung, and Blood Institute. It is general information, not medical advice. Scoring rules and insurance definitions can vary; ask your clinician to explain your individual report.

Byline: Sleep Clinic Directory Editorial Team. Last updated October 27, 2026.