Does Sleeping at Irregular Times Increase Heart Disease Risk?
In this study, yes, and the effect showed up in how much sleep varied night to night rather than in how much sleep people got. Among 1,992 adults followed for a median of 4.9 years, those whose sleep duration swung by more than two hours from night to night had 114% higher risk of a cardiovascular event than those who stayed within an hour. The finding held after adjusting for average sleep duration, so it is not a story about short sleep.
Dr. Kumar’s Take
What makes this study worth reading is the measurement. Sleep was recorded with seven days of wrist actigraphy rather than a questionnaire, so the irregularity is observed rather than remembered. Self-reported sleep is unreliable in exactly the way that matters here, because people report their typical night and irregularity is precisely the absence of a typical night.
The dose-response pattern is what moves me on this one. Risk climbs step by step across the categories of variability, for both sleep duration and sleep timing, with a significant trend for each. A single high-risk group can be an artifact. A ladder is harder to explain away.
Two caveats I would not skip. This is observational, so irregular sleep may be marking something else about a person’s life or health rather than causing the events. And 111 events across 1,992 people is not many, which is why the confidence intervals on the middle categories are wide enough to include no effect at all. The signal sits in the extremes and in the trend, not in fine distinctions between adjacent groups.
Still, the practical implication is unusually cheap to act on. Most sleep advice asks you to find more hours, which most people cannot. This asks you to put the hours in roughly the same place each night, which many people can.
What the Study Found
Of 1,992 participants, 111 had a cardiovascular event during follow-up, an overall rate of about 5.6% over a median 4.9 years.
Variability in how long people slept, measured as the standard deviation across seven nights. The absolute event rates come first, because they are what the numbers mean:
- 60 minutes or less: 8.8 events per 1,000 person-years (reference)
- 61 to 90 minutes: 9.1 per 1,000 person-years (9% higher risk, HR 1.09, 95% CI 0.62 to 1.92, not statistically significant)
- 91 to 120 minutes: 14.1 per 1,000 person-years (59% higher risk, HR 1.59, 95% CI 0.91 to 2.76, not statistically significant)
- More than 120 minutes: 19.3 per 1,000 person-years (114% higher risk, HR 2.14, 95% CI 1.24 to 3.68)
- Test for trend across categories: p = 0.002
In plainer terms, the most irregular sleepers had about 19 cardiovascular events per 1,000 people per year against about 9 in the most regular, so roughly 10 extra events per 1,000 people per year.
Variability in what time people fell asleep, compared against 30 minutes or less:
- 31 to 60 minutes: 16% higher risk (HR 1.16, 95% CI 0.64 to 2.13), not statistically significant
- 61 to 90 minutes: 52% higher risk (HR 1.52, 95% CI 0.81 to 2.88), not statistically significant
- More than 90 minutes: 111% higher risk (HR 2.11, 95% CI 1.13 to 3.91)
- Test for trend across categories: p = 0.002
Excluding current shift workers produced similar results, which matters because shift work is the obvious confounder for any finding about circadian disruption.
Study Design
This was a prospective cohort analysis within the Multi-Ethnic Study of Atherosclerosis. Participants were free of cardiovascular disease at baseline and completed seven days of wrist actigraphy between 2010 and 2013, then were followed through 2016. Sleep regularity was quantified as the standard deviation of actigraphy-measured sleep duration and of sleep-onset timing across those seven days. Incident cardiovascular disease included both nonfatal and fatal events.
A Cox proportional hazards model estimated risk by category of variability, adjusted for traditional cardiovascular risk factors including cardiovascular biomarkers, and for other sleep-related factors including average sleep duration.
Why Regularity Might Matter Separately From Duration
The cardiovascular system runs on strong circadian rhythms, and blood pressure, heart rate and vascular function all follow daily cycles. Shift work is the extreme case of disrupting those cycles and has long been linked to cardiovascular risk. The argument this study makes is that irregular sleep schedules are a milder version of the same disruption, and a far more common one, because they do not require an unusual job.
That framing explains why the adjustment for average sleep duration matters so much. If irregularity carried risk only by producing short nights, the association should have weakened once duration was accounted for. It did not.
How to Apply This
- Aim for a consistent bedtime, not just enough hours: the variability measured here was night-to-night swings in duration and onset time
- Watch the two-hour threshold: the clearest signal was in people whose sleep duration varied by more than two hours across a week
- Treat weekend catch-up as a cost: sleeping in on weekends is one of the more common sources of a large duration swing
- Keep the wake time steady if you can only fix one thing: a stable wake time anchors both timing and duration
- Raise it with your physician if you work shifts: the finding held after excluding shift workers, so it adds to rather than replaces that risk
Limitations To Keep In Mind
This is observational, so it establishes association rather than cause, and irregular sleep may mark other things about health, work or circumstance that carry the actual risk. There were 111 events in 1,992 participants, which leaves the middle categories with confidence intervals wide enough to include no effect. Sleep was measured over a single seven-day window, which may not represent someone’s habits across years of follow-up. The absolute rates above are for this cohort, which was free of cardiovascular disease at baseline and had a median age in the sixties, so they do not transfer directly to a younger or higher-risk person.
Related Studies And Internal Links
FAQs
How irregular is too irregular?
In this study, risk rose significantly once sleep duration varied by more than two hours night to night, or once sleep-onset time varied by more than 90 minutes.
Does this mean sleep duration does not matter?
No. It means irregularity carried risk independently of duration. The analysis adjusted for average sleep duration and the association held.
Is this just about shift workers?
No. Excluding current shift workers produced similar results, which is one of the more useful things about the analysis.
Does fixing my schedule lower my risk?
This study cannot answer that. It observed an association over five years rather than testing an intervention, so a trial would be needed to show that changing your schedule changes outcomes.
Conclusion
Among 1,992 adults tracked with wrist actigraphy and followed for a median of 4.9 years, greater night-to-night variability in both sleep duration and sleep timing was associated with higher cardiovascular risk, with the highest-variability groups at roughly double the risk of the most regular sleepers and a significant trend across categories for each measure. The association survived adjustment for average sleep duration and for traditional cardiovascular risk factors, and persisted after excluding shift workers. It remains observational, and 111 events is a modest number, so this belongs in the column of reasons to keep a steady schedule rather than proof that irregularity causes heart disease.
