A new study from the University of Oulu in Finland has found that adults with inconsistent bedtimes face roughly twice the risk of major cardiac events compared to those who go to bed at consistent times. The research was published in BMC Cardiovascular Disorders.
The study tracked 3,231 people born in Northern Finland in 1966. Researchers monitored their sleep patterns using activity trackers over one week when participants were 46 years old. Their health outcomes were then followed for more than a decade through national healthcare registry data.
During the follow-up period, 128 participants, or 4 percent of the total, experienced a major adverse cardiac event. These included acute myocardial infarction, unstable angina, stroke, and heart failure hospitalizations.
The heightened risk was specific to participants who also slept less than the group median of 7 hours and 56 minutes per night. Among that group, those with irregular bedtimes had a 2.01-fold higher risk of a major cardiac event compared to those with consistent bedtimes. Those with irregular sleep midpoints, defined as the midpoint between falling asleep and waking up, showed a 2.00-fold higher risk.
Why Bedtime Matters More Than Wake-Up Time for Heart Health
One of the key findings separates this study from earlier sleep research. The researchers looked independently at bedtime variability, wake-up time variability, and sleep midpoint variability. Only bedtime and sleep midpoint irregularity showed a significant link to cardiac risk. Irregular wake-up times showed no clear association with major cardiovascular events.
Postdoctoral researcher Laura Nauha from the University of Oulu, the study’s lead author, said this is the first study to examine these three variables separately and track their independent associations with cardiac outcomes. “Our findings suggest that the regularity of bedtime, in particular, may be important for heart health. It reflects the rhythms of everyday life and how much they fluctuate,” Nauha said.
The study adjusts for several variables that could influence cardiac outcomes, including gender, body mass index, systolic blood pressure, glycated hemoglobin, LDL cholesterol levels, employment status, and total physical activity.
Nauha noted that while many factors affecting heart health are outside an individual’s control, sleep timing is one that most people can actively work to regulate. “Maintaining a regular sleep schedule is one factor that most of us can influence,” she said.
What the Irregular Sleep and Cardiovascular Disease Research Means
The study defines irregular sleep timing using a seven-day standard deviation for each sleep variable. Participants were divided into three groups: regular, fairly regular, and irregular. The association between irregular bedtime and elevated cardiac risk only emerged in those sleeping below the group median, suggesting that short sleep duration and inconsistent timing together compound the cardiovascular risk.
The research did not establish a direct causal mechanism, but the authors note that sleep timing is closely tied to the body’s circadian system, which regulates cardiovascular processes including heart rate, blood pressure, and vascular tone. Disrupting the timing of sleep is understood to interfere with these circadian rhythms.
The study’s cohort was drawn from a single birth year in Northern Finland, which may limit how broadly the results apply to other populations. Researchers also relied on wearable device data to estimate time in bed rather than directly measuring sleep onset and offset.
The findings add to a growing body of research connecting sleep behavior to heart health, and point to bedtime consistency, not just total sleep duration, as a factor worth considering in cardiovascular health guidelines.
