A growing body of evidence suggests that irregular bedtimes, and not just sleep length, may carry hidden cardiovascular risks.
Researchers at the University of Oulu found that large variations in bedtime could double the likelihood of serious cardiac events, especially among people who sleep less than eight hours per night.
In the study published in BMC Cardiovascular Disorders, the researchers reported that irregular bedtimes and greater variation in sleep timing were strongly connected to an increased risk of major cardiovascular events.
This link was most evident in participants whose time in bed was under eight hours, where the risk was about twice as high compared with those who followed more consistent sleep schedules.
In contrast, inconsistent wake-up times did not show a clear association with heart health. Major cardiovascular events in the study referred to conditions requiring specialised medical care, such as myocardial infarction or cerebral infarction.
The finding places the focus on repeated habit, not on whether someone simply prefers early mornings or late ones.
The study tracked 3,231 individuals born in Northern Finland in 1966. Their sleep patterns were measured over one week at age 46, and their health outcomes were followed for more than a decade using healthcare registry data.
Researchers estimated sleep duration and timing using activity monitors that recorded how long participants spent in bed.
Participants with irregular bedtimes who slept less than eight hours had twice the risk of myocardial infarction (heart attack) or cerebral infarction (stroke) compared with those with stable schedules.
It found that the “anchor” for heart health is the time one goes to sleep, not necessarily the time one wakes up. Variability in the “sleep midpoint” (the halfway point between falling asleep and waking up) was also strongly associated with higher cardiac risk.
Within that shorter-sleep group, irregular bedtimes carried about twice the event risk, and the night’s midpoint showed nearly the same pattern.
Previous research has linked irregular sleep patterns to heart health risks, but this is the first study that looked separately at variability in bedtime, wake-up time and the midpoint of the sleep period and major cardiac events.
Postdoctoral researcher Laura Nauha from the University of Oulu, who was involved in the study, declared: “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 that many factors influencing heart health are tied to daily habits, stating: “Maintaining a regular sleep schedule is one factor that most of us can influence.”
Consistent sleep keeps the body’s circadian rhythms, its internal 24-hour clock, in sync with light, hormone cycles and blood pressure. When bedtime shifts from night to night, that timing can become disrupted, placing added strain on the heart during hours normally reserved for rest.
Shorter nights can worsen the strain because the body loses both recovery time and the steady schedule that guides it. This biological explanation does not prove causation in this study, yet it fits what heart researchers already know.
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