Adults who went to bed at wildly different times from one night to the next faced nearly double the risk of heart attack compared with those who kept steadier schedules, according to findings from two large cohort studies. The research, which tracked participants for up to a decade using wrist-worn accelerometers, adds weight to a growing body of evidence that erratic sleep timing is not just a lifestyle inconvenience but a measurable cardiovascular hazard. With heart disease still the leading killer of Americans, the results put a spotlight on a low-cost behavior that millions of people could change starting tonight.
Why erratic bedtimes are drawing attention from cardiologists
The connection between sleep and heart health has long centered on duration: too little or too much sleep raises red flags. But newer research shifts focus to consistency. According to the National Institutes of Health, the Multi-Ethnic Study of Atherosclerosis (MESA) enrolled nearly 2,000 participants ages 45 to 84 and tracked their sleep patterns with wrist accelerometers rather than self-reported diaries. That objective measurement matters because people routinely misjudge when they fall asleep and wake up.
The MESA team tested whether irregular sleep timing and irregular sleep amounts predicted heart attack, stroke, and related cardiovascular events. The association held even after the researchers accounted for total sleep duration, meaning that two people who each averaged seven hours a night could face very different cardiac risks depending on how much their bedtimes bounced around. For midlife adults wondering whether a consistent schedule is worth the effort, the data offers a concrete answer: variability itself appears to carry independent risk.
A separate question, and one no trial has yet answered, is whether deliberately reducing bedtime variability by a meaningful margin, say 30 minutes or more, would produce a measurable drop in major adverse cardiac events over five years. That hypothesis remains untested in a randomized setting, but the observational signal is strong enough that sleep researchers are calling for exactly that kind of intervention study.
Accelerometer data from two cohorts point in the same direction
The MESA findings, published in the Journal of the American College of Cardiology, drew on a diverse U.S. population and recorded cardiovascular events over several years of follow-up. The study used accelerometer data for sleep metrics, removing much of the guesswork that plagues questionnaire-based research. Greater irregularity in both sleep timing and sleep duration was linked to a higher incidence of heart attack and stroke, with the most irregular sleepers facing roughly double the risk of the most regular ones.
A second, more recent cohort study reinforced those results from a different angle. In a Northern European sample of midlife adults, more than 3,000 participants were followed for about a decade while researchers objectively measured sleep-timing irregularity, including bedtime variability. The outcome tracked was incident major adverse cardiac events and cardiovascular disease mortality, a hard clinical endpoint that goes beyond surrogate markers like blood pressure or cholesterol levels.
The two studies differ in important ways. The MESA cohort was multi-ethnic and U.S.-based, while the Northern cohort drew from a more geographically and culturally homogeneous population. The NIH summary describes nearly 2,000 participants, while the European study reports 3,231. These are separate investigations, not conflicting counts from the same project. Their convergence on the same general finding-that bedtime irregularity predicts cardiac events-strengthens the case that the association is not an artifact of one particular sample or measurement protocol.
Both studies relied on accelerometers rather than sleep logs, which is significant because objective measurement reduces recall bias. People who work rotating shifts, care for young children, or simply scroll their phones past midnight on weekends may not realize how much their bedtimes drift. The accelerometer data captures that drift precisely, down to the minute, allowing researchers to quantify variability instead of relying on rough estimates.
What the statistics can and cannot tell us
Strong as the signal is, several gaps remain. Neither study was a randomized trial, so the data cannot prove that irregular bedtimes cause heart attacks rather than simply traveling alongside other risk factors. Shift workers, for instance, tend to have more variable schedules and also face higher rates of obesity, metabolic syndrome, and limited access to preventive care. The published analyses adjusted for known confounders like smoking, physical activity, and sleep duration, but participant-level data on socioeconomic status and detailed shift-work history are not fully extractable from the available records.
More broadly, sleep is tightly interwoven with other behaviors. People who keep very irregular hours may also have inconsistent meal times, greater alcohol intake, or higher stress levels. Any of these could contribute to cardiovascular risk. Statistical adjustments can only go so far when many of these factors overlap, and residual confounding is difficult to rule out.
Physiologically, researchers suspect that irregular sleep schedules may disrupt circadian rhythms that govern blood pressure, autonomic nervous system activity, and metabolic processes. Over years, that disruption could promote atherosclerosis or increase the likelihood of plaque rupture. But mechanistic pathways are still being clarified, and the current evidence is strongest at the epidemiologic level.
How much irregularity matters?
One of the most practical questions for clinicians and patients is how much variability is too much. The MESA analysis, detailed in a peer-reviewed open-access report, grouped participants by standard deviation of their sleep timing and duration. Those in the highest categories of variability had the greatest risk, suggesting a dose–response pattern rather than a simple threshold effect. Yet the studies were not designed to define a precise cutoff that could be translated into a clinical guideline.
Direct statements from the study authors on specific recommendations-such as whether keeping bedtimes within a 30-minute window is substantially safer than a 90-minute swing-are limited. That leaves clinicians in a familiar position: the observational evidence is persuasive, but the prescription is still somewhat vague. Telling a patient to “go to bed at the same time every night” is easy advice to give and hard advice to follow, especially for people whose work or family obligations make rigid schedules impractical.
Another open question is how quickly risk might change if someone improves their sleep regularity. Because the cohorts mainly assessed variability over a defined baseline period and then followed participants forward, they say little about what happens when people shift from erratic to consistent schedules. Longitudinal intervention trials, in which participants actively work to regularize their sleep and are followed for cardiovascular outcomes, would be needed to answer that.
What people can do now
Despite these uncertainties, the studies point toward several pragmatic steps. First, adults who already aim for seven to eight hours of sleep may want to pay as much attention to when they sleep as to how long. That could mean setting a target bedtime and wake time for most days of the week, including weekends, and adjusting social or work commitments where possible to protect that window.
Second, people whose schedules are inherently variable-such as rotating-shift workers-may benefit from minimizing unnecessary swings. Even if work demands late nights on some days, avoiding large, discretionary shifts on off days could reduce overall variability. Simple tools like calendar reminders, dimming lights at a consistent hour, and limiting late-evening screen use can help anchor a more regular routine.
Finally, clinicians may wish to incorporate questions about sleep timing regularity into cardiovascular risk assessments, alongside traditional factors like blood pressure, cholesterol, and smoking. While it is too early to assign a specific risk score to bedtime variability, identifying highly irregular patterns could open the door to counseling that, at minimum, is unlikely to cause harm and may offer meaningful benefit.
For now, the message emerging from accelerometer-based studies is straightforward: when it comes to protecting the heart, it is not just how much you sleep that matters, but also how predictably you do it. In a landscape where many risk factors are costly or difficult to modify, going to bed at a more consistent hour stands out as a change that is both accessible and, increasingly, backed by data.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
