What Are RERAs and Why Do They Matter Beyond AHI?

RERA stands for Respiratory Effort-Related Arousal. In plain language: it is a period of increased breathing effort that ends in an arousal — a brief waking moment — without crossing the threshold for a full apnea or hypopnea.

During normal breathing, your airway stays open and your respiratory effort is low and consistent. During an obstructive apnea, the airway closes completely for at least 10 seconds. During a hypopnea, airflow drops significantly (typically 30% or more) with an associated oxygen desaturation or arousal.

A RERA fits neither definition. It might be:

• A partial narrowing of the airway that causes increasing respiratory effort
• A gradual rise in breathing resistance that your brain detects and wakes from
• A pattern of flow limitation that doesn't quite dip enough to score as a hypopnea

What they share: increased effort, a disruption to sleep continuity, and no trace in your AHI.

It helps to think of these three event types as a spectrum of airway obstruction severity.

This is not a flaw in your device. It reflects how these metrics were defined — and the definitions have changed over time.

The AHI (Apnea-Hypopnea Index) counts apneas and hypopneas per hour of sleep. By definition, RERAs are excluded.

The AASM (American Academy of Sleep Medicine) scoring guidelines have changed over time, and different labs use different hypopnea scoring rules. Under some rulesets, events that would be counted in AHI are scored as RERAs under others. Two people with identical underlying breathing patterns can receive very different AHI numbers depending on which criteria were applied.

The practical result: someone using CPAP therapy with a low AHI might still have significant respiratory effort going on throughout the night — it is just not being counted in the headline metric.

Some sleep specialists use the RDI (Respiratory Disturbance Index) instead, which includes RERAs alongside apneas and hypopneas. But most CPAP machines do not report RDI. Your device summarises your night as AHI, and RERAs remain invisible in that number.

This is where it gets interesting if you look at your raw CPAP data. RERAs have a characteristic signature in the airflow waveform: flow limitation. Instead of the normal rounded shape of a healthy breath, flow-limited breaths have a flattened top — the airway is partially narrowed, airflow plateaus early, and the breath looks truncated rather than arched.

A sequence of flow-limited breaths with increasing effort, followed by an abrupt shift in breathing pattern (the arousal clearing the obstruction), is the fingerprint of a RERA in the raw data.

You will not see this in your device's summary numbers. But it is in the detailed flow waveform data that your CPAP or BiPAP records to its SD card — if you know how to look.

For a deeper dive into flow limitation patterns specifically, see our article Understanding Flow Limitation in CPAP Data — it covers how to identify these waveform shapes in detail.

AirwayLab reads your SD card data directly in your browser. Your data never leaves your device. It gives you access to the same breath-by-breath flow data available in tools like OSCAR, with visualisations designed to make flow limitation and RERA-related patterns easier to explore.

Here is what to look for when investigating your nightly data:

This is not diagnosis — it is data. What you see in AirwayLab gives you something concrete to discuss with your clinician: specific timestamps, specific flow patterns, a picture of what your nights actually look like beyond the headline number.

If you are new to reading CPAP data, How to Read Your CPAP Data and How to Export and Understand Your CPAP Data are good starting points.

Your AHI is a useful summary. It is not a complete picture. RERAs are one of the reasons a low AHI does not automatically mean fully restorative sleep — there are others, including periodic limb movements, central breathing patterns, and oxygen-independent arousals.

The point of looking at your data beyond AHI is not to self-diagnose or second-guess your therapy settings. It is to understand what is actually happening in your airway during the night, and to have a more informed conversation with whoever is managing your care.

AirwayLab is free and always will be. It runs entirely in your browser with no account required, and the core engine is published under GPL-3.0. Open your SD card in AirwayLab to see your RERA-type pattern estimates alongside AHI.

Three numbers often come up in sleep apnea discussions — AHI, RDI, and RERA index. They are related but measure different things:

Your CPAP machine reports AHI. It does not report RDI. So if your AHI is 1.5 but your RERA index is 8, your RDI could be close to 10 — a number your nightly summary never shows you.