Still tired on CPAP? How common it is, and what the studies say

Your CPAP estimates events from the airflow signal in the mask. When airflow stops or drops for long enough, the machine counts an apnea or a hypopnea and divides by hours of use. That gives the AHI on your screen. For the definitions, see AHI explained.

A sleep lab records more, including brain activity (EEG), oxygen levels and chest effort. Sleep studies also score arousals, brief wake-ups seen on EEG. Your machine has no EEG, so it has no way to score them.

How well the two numbers agree depends on the device and the setting. In an in-lab study of 137 people the device AHI and the lab AHI were almost identical on average (3.35 versus 3.40) [S5]. In a study of 70 people the device read higher than the lab (9.9 versus 4.2), mainly because of hypopneas, and higher leak was linked to a bigger gap [S4]. Both were lab studies, so home results can differ.

What this means for you: the machine AHI describes breathing events under your mask. It does not describe how well you slept.

Numbers vary with who is counted and when.

• In a Danish national registry of 1,174 people with sleep apnea, 15.6% still had excessive sleepiness after 1 to 13 months of CPAP [S2]. That is roughly 1 in 6. Two authors are current or former employees of a drug company, as disclosed in the paper.
• In the European Sleep Apnea Database, 28.2% of 4,853 people scored above 10 on the Epworth sleepiness scale at a median of 5 months [S1]. The rate was 40% in the first 3 months and 13 to 19% for people first followed up between 4 months and 2 years [S1]. What this means: the first weeks look worst, and the later figure is closer to 1 in 7 or 1 in 5.
• In a French registry, 13% of CPAP users had residual sleepiness after excluding people who used CPAP under 3 hours, had a residual AHI above 15, or had major depression [S3]. Among people in that same group using CPAP 6 hours or more it was 9% [S3]. What this means: even with a residual AHI of 15 or below and regular use, a smaller group stays sleepy.

The first two figures include people with a high residual AHI or low use. Only the French study removes them. It found 13% for a residual AHI of 15 or below with at least 3 hours of use, and 9% (about 1 in 11) for those using CPAP 6 hours or more [S3]. The share is smaller, but it is real.

In the European database, a residual AHI above 10 together with nightly use under 4 hours occurred about equally often in people with and without sleepiness at follow-up [S1]. Use still mattered in the other two studies: in the Danish registry, people who kept to their CPAP were less likely to stay sleepy [S2], and in the French registry the rate fell from 13% to 9% among people using CPAP 6 hours or more [S3]. The European authors also saw differences between countries and link them to cultural and lifestyle factors [S1]. A single machine number does not capture all of that.

These are things that can make a conversation with your clinician more concrete. Your clinician decides what, if anything, to do with them.

• Usage hours. How long you wear the mask each night. A good average can hide a few very short nights. What this means: it shows how much of the night the machine covered.
• Leak. Air escaping around the mask. Higher leak was linked to a bigger device-versus-lab gap in one study [S4]. The AASM guideline lists use, residual severity and leak together as what remote PAP monitoring covers [S6]. See CPAP leak rate explained. What this means: it may affect how much to trust the event counts on a given night.
• Flow limitation. Flattened breaths that may not qualify as a hypopnea, so they may not be counted in the AHI. See understanding flow limitation. What this means: it is a pattern to show your clinician, and it is not a diagnosis.
• The waveform. The breath-by-breath trace shows what the summary hides, such as clusters of events, mask-off periods and changes across the night. Free tools such as OSCAR show it, and so does AirwayLab. What this means: it lets you and your clinician look at the same night together.

If flow limitation looks high on a night you felt awful, bring that to your clinician. Settings are a clinical decision for them.

If you feel tired on treatment, tell your clinician, ideally with a few weeks of data. They can look at the full picture, which a machine summary does not cover.