If you've spent any time in CPAP forums, you've probably heard someone say their AHI is fine but they still feel exhausted during the day. RERAs — Respiratory Effort-Related Arousals are often part of that picture. They are real events in your breathing data that your machine's nightly summary quietly ignores.
This article explains what are RERAs sleep apnea events, why they don't show up in your AHI score, how they look in raw CPAP flow data, and how AirwayLab can help you see them in your own nightly recordings.
Medical disclaimer: This article is for informational purposes only. AirwayLab is not a medical device, and nothing here constitutes a diagnosis or treatment recommendation. Always discuss your therapy data with a qualified clinician before making any changes to your settings or care.
What Is a RERA?
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.
How RERAs Differ from Apneas and Hypopneas
It helps to think of these three event types as a spectrum of airway obstruction severity.
Apnea
A near-complete stop in airflow lasting at least 10 seconds. Obstructive apneas happen when the airway physically collapses. Central apneas happen when the brain temporarily stops sending the signal to breathe. Both are counted in your AHI.
Hypopnea
A partial reduction in airflow — typically 30% or more below baseline — lasting at least 10 seconds, accompanied by either an oxygen desaturation or an arousal from sleep. Hypopneas are also counted in your AHI.
RERA
Subtler than both. Airflow may drop, but not enough to score as a hypopnea under the applicable criteria. What defines it is the pattern of increasing respiratory effort followed by an arousal that clears the obstruction. The effort is there in the flow waveform. The arousal is there in the EEG (in a sleep lab) or detectable as a breathing-pattern shift in CPAP data. But the event does not cross the AHI threshold.
This is not a flaw in your device. It reflects how these metrics were defined — and the definitions have changed over time.
Why RERAs Don't Appear in Your AHI
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.
How RERAs Show Up in Flow Data
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.
Seeing RERA-Related Patterns in AirwayLab
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:
Flow waveform view
Load your session and look at the detailed flow channel. Healthy breaths appear as smooth arcs. Breaths during flow limitation look flatter on top — the plateau is the tell. A run of flattened breaths followed by a larger, more forceful breath is consistent with a RERA-type pattern.
Flow limitation channel
AirwayLab surfaces your device's flow limitation score alongside the raw waveform. Look for periods where flow limitation is elevated but your event markers show no scored apneas or hypopneas. That gap — flagged limitation with no scored events — is exactly where RERA-type patterns tend to live in the data.
Clustering by time of night
RERAs related to body position (for example, sleeping on your back) often cluster in specific windows. AirwayLab's timeline view lets you see whether patterns are consistent across sessions and correlate with time.
Multi-session comparison
If you want to understand whether changes in your therapy affect these patterns, AirwayLab's multi-session view lets you compare flow limitation trends across nights rather than relying on AHI alone.
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.
AHI Is One Number
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 source code is publicly available under GPL-3.0 — so you can verify exactly what it does with your data.
Explore What Your AHI Might Be Missing
Upload your SD card data and look at the flow limitation channel — the part of your data most closely linked to RERA-type patterns.
Related reading
Understanding Flow Limitation in CPAP Data — the waveform patterns behind RERA-type events.
How to Read Your CPAP Data — a beginner's guide to the metrics that matter.
Why Your AHI Is Lying to You — a deep dive into AHI's limits as a sleep quality metric.
AirwayLab is a free, open-source tool for analysing PAP flow data. Your data never leaves your browser. Nothing on this page constitutes medical advice -- always discuss your results with a qualified sleep specialist.