The Difference Between an Apnea a Hypopnea and a Breathing Related Arousal
How apnea vs hypopnea and a breathing-related arousal differ on your sleep report
Your sleep report names three airway events as if you already know them. This guide shows how clinicians tell an apnea, a hypopnea and a breathing-related arousal apart.
Apnea vs hypopnea vs arousal, the short answer
The short answer is that an apnea is a near-complete stop in airflow, a hypopnea is a partial drop, and a breathing-related arousal is a brief brain arousal that follows rising breathing effort. The apnea vs hypopnea split comes from the airflow signal, while the arousal is scored from effort and brain waves. All three can break up your sleep, which is why a report can count them.
Your report turns these events into rates per hour. The headline number is the apnea-hypopnea index (AHI), and our guide to what your AHI score actually means covers the severity bands.
Breathing-related arousals sit outside the AHI, so a calm number can hide broken sleep. Watch for these signs.
- Breathing pauses a partner has witnessed
- Excessive daytime sleepiness after a full night in bed
- Morning headaches
- Repeated awakenings you cannot explain
- Apnea is a near-total airflow stop, hypopnea a partial one.
- A breathing-related arousal is rising effort that ends in a brief brain arousal, not a full awakening.
- The AHI counts only the first two.
How a sleep study tells the three events apart
A sleep study reads several sensors at once, and for apneas and hypopneas the airflow sensor does most of the work.
- A nasal pressure sensor tracks the airflow reduction from baseline.
- Effort belts, or thoracoabdominal bands, show whether the chest and belly keep working.
- A finger clip reads SpO2, your blood oxygen saturation.
- EEG electrodes pick up arousals. Full polysomnography (PSG) records them, but most home tests do not.
The rules come from the AASM scoring manual. The Sleep Foundation (2026) describes an apnea as a near-complete halt in airflow lasting at least 10 seconds. A hypopnea needs a clear partial reduction in airflow from the pre-event baseline, again for at least 10 seconds. Under the recommended AASM rule (2020), it also needs a small drop in blood oxygen or an arousal. An older, stricter rule asks for a larger oxygen drop and no arousal, and labs still use both, as a 2019 study in Cureus reports.
A RERA, short for respiratory effort-related arousal, is at least 10 seconds of increasing effort that ends in an arousal. It meets neither rule above and does not require any oxygen drop, as the definition behind the respiratory disturbance index spells out.
The belts also settle the central vs obstructive event question. In an obstructive event the chest keeps pulling against a narrowed throat. In a central event the effort pauses too.
What arousal means on your report
An EEG arousal is a short burst of faster brain-wave activity lasting a few seconds. This cortical arousal is not a full awakening, so you rarely remember it. Many in one night add up to sleep fragmentation.
| Event | Main signal | Oxygen drop needed | In the AHI | In the RDI |
|---|---|---|---|---|
| Apnea | Airflow nearly stops for 10 seconds or more | Not required | Yes | Yes |
| Hypopnea | Airflow falls partway for 10 seconds or more | Yes, or an arousal under the recommended rule | Yes | Yes |
| RERA (breathing-related arousal) | Effort rises for 10 seconds or more, then an EEG arousal | Not required | No | Yes |
- Apneas and hypopneas are scored from airflow.
- A RERA is scored from effort plus an EEG arousal.
- Each event needs a 10-second duration at minimum.
Why two labs can score the same night differently
Two labs can give different AHIs for one night. One common reason is that two hypopnea rules are in use. The recommended AASM rule, written H3A, accepts the smaller oxygen dip or an arousal. The acceptable rule, H4, needs the larger dip and does not count arousals.
European practice that follows the AASM recommendation uses H3A, while H4 survives mainly in US insurance rules. Any event that meets H4 also meets H3A, so an H3A score typically shows the same or a higher AHI. The split also explains conflicting hypopnea numbers online. If two reports disagree on the hypopnea count or the AHI, check which rule each lab used before deciding your breathing has changed.
Reading a French, Spanish or UK report
European reports often use local names.
- France: SAHOS (syndrome d'apnées-hypopnées obstructives du sommeil) and IAH (indice d'apnées-hypopnées), the French AHI. According to the Haute Autorité de Santé, diagnosis is often a home polygraphie ventilatoire rather than full polysomnography.
- Spain: IAH and SAHS, the local names for the AHI and the syndrome.
- UK: the apnoea spelling and the OSAHS label, short for obstructive sleep apnoea-hypopnoea syndrome. NICE guideline NG202 (2021) grades the AHI as mild from 5 to 14, moderate from 15 to 30 and severe above 30.
A polygraphie ventilatoire, like most home sleep apnea tests, does not usually record brain waves. It may miss arousal-based hypopneas and RERAs that a full PSG would score.
- H3A and H4 can give different AHIs for one night.
- IAH is the French and Spanish AHI.
- Home polygraphy may not score arousals.
Why keeping the upper airway open matters
Obstructive apneas and hypopneas share one mechanism. The upper airway narrows or closes during sleep, and airflow falls. How much air still gets through decides the label. So the sensible focus is keeping the upper airway open and maintaining enough airflow.
Back2Sleep is one option that some people with snoring or mild to moderate obstructive sleep apnea (OSA) explore. It is a soft silicone intranasal stent, a CE-certified Class I medical device that keeps the nasal airway open during sleep, with no prescription needed. It is less suitable when the blockage sits at the tongue base or at several levels. It is not meant for central events, severe apnea or RERAs from deeper airway resistance.
- Obstructive events depend on how much air gets through.
- A nasal stent supports only the nasal part of the airway.
- Severe or central apnea needs a plan from your doctor.
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Frequently Asked Questions
What is the difference between an apnea and a hypopnea?
An apnea is a near-complete stop in airflow lasting at least 10 seconds. A hypopnea is a partial drop in airflow from baseline over the same time, confirmed by an oxygen dip or an arousal (AASM, 2020). Both are scored from the airflow signal, and both count toward your AHI.
Is a hypopnea as serious as an apnea?
The evidence suggests it is. A 2019 Cureus study found no clinically meaningful difference between apneas and hypopneas, echoing earlier work where patients with recurrent hypopneas were clinically indistinguishable from those with recurrent apneas. A report dominated by hypopneas therefore deserves the same attention as one full of apneas.
What is a RERA in a sleep study?
A RERA, or respiratory effort-related arousal, is at least 10 seconds of rising breathing effort that ends in a brief brain arousal. It does not meet the airflow rules for an apnea or a hypopnea, and it does not require an oxygen drop. RERAs feed the RDI rather than the AHI.
Why is my AHI normal but I am still tired?
The AHI counts only apneas and hypopneas. If rising breathing effort keeps ending in arousals, those RERAs can raise the RDI while the AHI stays under 5. This pattern appears in upper airway resistance syndrome. Ask whether RERAs were scored, and describe your daytime sleepiness to your doctor.
What is the difference between AHI and RDI?
The AHI, or apnea-hypopnea index, counts apneas and hypopneas per hour of sleep. The RDI, or respiratory disturbance index, adds RERAs on top. Because the RDI includes an extra event type, it is equal to or higher than the AHI. A clear gap between the two points to breathing-related arousals.
Does a home sleep apnea test detect RERAs?
Often not. Most home tests, including a French polygraphie ventilatoire, record airflow, effort and oxygen but not brain waves. Scoring a RERA needs an EEG arousal, so home studies tend to miss them and can under-count a real breathing problem. A full polysomnography in a sleep lab can capture these arousals.
How long does a breathing event have to last to count?
Each of the three events must last at least 10 seconds to be scored. An apnea needs a near-complete halt in airflow for that time, a hypopnea a partial drop, and a RERA a run of rising effort that ends in an arousal. Shorter pauses are not scored as breathing events.
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