A standard sleep apnea diagnosis works like this: you spend one night in a lab or wear a home monitor, and a number — the Apnea-Hypopnea Index — tells your doctor how severe your condition is. Mild, moderate, or severe. Treatment decisions follow.
But a study published in the journal SLEEP by researchers at Flinders University in Adelaide, Australia, suggests that single snapshot may be dangerously incomplete. People whose sleep apnea fluctuates sharply from one night to the next face significantly higher cardiovascular risk — regardless of where their average severity falls.
The Study
Bastien Lechat, Ph.D., and colleagues at Flinders University analyzed multinight sleep data from 3,159 participants (19% female, average age 49, average BMI 29). Rather than relying on a single night's recording, the researchers tracked how much each person's AHI varied across multiple nights of monitoring.
Among the participants, 142 (4.5%) had experienced a major adverse cardiac or cerebrovascular event (MACCE) — a composite measure that includes heart attack, stroke, angina, coronary artery disease, and congestive heart failure.
Variability as an Independent Risk Factor
The central finding: participants with high night-to-night variability in their AHI (at the 75th percentile, swinging by about 8 events per hour between nights) had 34% higher odds of having a MACCE compared to those with stable patterns (at the 25th percentile, varying by only 2.8 events per hour).
That 34% increase held after adjusting for overall OSA severity, age, sex, BMI, and other cardiovascular risk factors. In other words, the volatility itself — not just the average number of breathing interruptions — appears to carry independent cardiac risk.
Why One Night Isn't Enough
The implications for current diagnostic practice are stark. Most sleep apnea evaluations rely on data from a single night, whether in a lab or at home. A patient whose apnea happens to be mild on testing night may be classified as low-risk and left untreated or under-treated — even if their condition is severe on other nights.
The Flinders team found that people with mild average apnea but high nightly variability had vascular health profiles resembling those with severe, stable apnea. Standard testing would flag the severe group for treatment but potentially miss the volatile mild group entirely.
"Sleep should be seen as a moving picture, rather than a single photograph," the researchers noted. Understanding nightly patterns could help doctors identify who needs early intervention.
A Companion Finding on Vascular Aging
A related paper from the same research group, published in npj Digital Medicine, found that multinight digital assessment of sleep-disordered breathing was associated with accelerated vascular aging — further reinforcing the idea that night-to-night instability in breathing carries biological consequences that a single-night average cannot capture.
Why Does Variability Cause Harm?
The researchers hypothesize that the cardiovascular system may be more damaged by unpredictable cycles of oxygen deprivation than by consistent, moderate exposure. Nights of severe desaturation interspersed with milder nights could trigger repeated surges in sympathetic nervous system activity, blood pressure spikes, and inflammatory responses — the kind of intermittent physiological stress that promotes arterial damage and clot formation.
This pattern echoes findings in blood pressure research, where variability in readings — not just the average level — has been established as an independent predictor of stroke and heart attack.
What This Means for Patients
For the estimated 30 million Americans with obstructive sleep apnea, the study raises a practical question: is your diagnosis based on a good night or a bad one?
Patients who have been told their apnea is "mild" but who experience symptoms that feel inconsistent — some mornings waking refreshed, others waking exhausted with headaches — may want to discuss multinight monitoring with their sleep physician. Home sleep testing devices that record multiple nights are increasingly available and may provide a more accurate picture.
For clinicians, the study suggests that AHI variability should be considered alongside AHI severity when stratifying cardiovascular risk. As multinight monitoring becomes more accessible through wearable technology and home-based testing, incorporating variability metrics into clinical decision-making could help identify a group of patients that current protocols miss.
