Most sleep advice focuses on how many hours you get. A study published in BMC Cardiovascular Disorders suggests the missing variable may be when you go to bed — and specifically, how much that timing drifts from one night to the next.
Researchers at the University of Oulu in Finland tracked 3,231 adults from the Northern Finland Birth Cohort 1966 for more than a decade after measuring their sleep patterns at age 46. The result: people whose bedtimes swung widely across the week faced roughly double the risk of a major cardiac event, including heart attack, stroke, and hospitalization for heart failure.
The surprise was what didn't matter. Irregular wake-up times showed no statistically significant connection to cardiac events at all.
What the Study Measured
Lead researcher Laura Nauha and colleagues fitted participants with wrist-worn actigraphy monitors for one week during their 46-year follow-up visit in 2012–2014. The devices recorded bedtime, wake-up time, and the midpoint of each sleep period. Researchers then calculated the standard deviation for each measure across the week and divided participants into three groups: regular, fairly regular, and irregular sleepers.
In the irregular group, bedtimes swung by an average of 108 minutes across the week. In the regular group, the swing was just 33 minutes.
Participants were then followed through Finnish health registers until December 31, 2023 — more than 10 years — tracking major adverse cardiac events (MACE), defined as myocardial infarction, cerebral infarction, unstable angina, hospitalization for heart failure, or cardiovascular death.
The Numbers
During follow-up, 128 participants (4.0%) experienced a major cardiac event. After adjusting for gender, employment status, BMI, blood pressure, blood sugar, cholesterol, and physical activity, the researchers found:
- Irregular bedtime was associated with a 2.01-fold higher risk of MACE (HR = 2.01, 95% CI: 1.00–4.01, P = 0.049)
- Irregular sleep midpoint carried a 2.25-fold higher risk (HR = 2.25, 95% CI: 1.01–5.01, P = 0.047)
- Irregular wake-up time showed no statistically significant link to cardiac events
The elevated risk was concentrated among participants who spent less than the study median of 7 hours and 56 minutes in bed. For this shorter-sleep group, irregular bedtime was associated with a 2.29-fold higher risk (HR = 2.29, 95% CI: 1.02–5.12, P = 0.044).
Why Bedtime, Not Wake-Up Time?
This is the first study to separately analyze bedtime, wake-up time, and sleep midpoint variability as independent predictors of cardiac events. Previous research had treated "sleep irregularity" as a single composite measure.
"Our findings suggest that the regularity of bedtime, in particular, may be important for heart health," Nauha said. "It reflects the rhythms of everyday life, and how much they fluctuate."
The biological explanation may involve the body's circadian clock, which synchronizes hormone release, blood pressure regulation, and inflammatory processes with the light-dark cycle. Irregular bedtimes may disrupt this clock in ways that irregular wake-up times — often compensated for by alarm clocks and morning routines — do not.
Dr. Brian Zachariah, a physician not involved in the study, noted that "irregular sleep timing and circadian disruption plays a role in metabolic and inflammatory dysregulation. It can cause insulin resistance, increase systemic inflammation, which in turn can elevate atherosclerosis and thrombosis."
Limitations Worth Noting
Sleep was measured over a single week at age 46, which may not fully capture long-term patterns. The study is observational and cannot establish that irregular bedtimes directly cause cardiac events. And the hazard ratios, while clinically meaningful, hover near statistical significance thresholds.
Still, the consistency of the finding across bedtime and sleep midpoint — and its absence for wake-up time — strengthens the case that something specific about bedtime regularity matters for cardiovascular health.
What This Means for Patients
For people already tracking their sleep duration, this study suggests a second metric worth watching: bedtime consistency. A person who sleeps seven hours but goes to bed at 10 p.m. one night and midnight the next may carry more cardiac risk than someone who consistently goes to bed at 11 p.m.
"Maintaining a regular sleep schedule is one factor that most of us can influence," Nauha said. "Protecting bedtime sounds simple, yet it points to something practical and possibly easier than changing many other risks."
The practical takeaway is straightforward: pick a bedtime, keep it within a narrow window most nights, and treat that consistency as a health behavior worth protecting — not just a matter of personal preference.
