An irregular sleep schedule in one’s 40s could quietly lay the groundwork for future heart problems. Researchers who observed thousands of people over a decade found that individuals with highly irregular bedtimes—especially if they slept less than eight hours—had about twice the risk of serious cardiovascular events such as heart attacks or strokes. Interestingly, it wasn’t primarily a matter of when people woke up, but rather how irregular their bedtime was.

Why Regular Bedtimes Might be Important for the Heart

If you go to bed at different times every night in middle age, this could be a warning sign for future heart problems. New research findings from the University of Oulu suggest that significant fluctuations in bedtime can substantially increase the risk of serious cardiovascular events, particularly among people who spend less than eight hours in bed. The study found a strong link between irregular bedtimes and an increased likelihood of serious cardiac events. People who slept less than eight hours and whose sleep patterns fluctuated significantly were at roughly twice the risk compared to those with more regular sleep habits. In contrast, irregular wake-up times showed no clear link to heart problems. The serious cardiovascular events in this study included conditions such as heart attack or stroke.

These are conditions in which the blood supply to vital organs is suddenly interrupted. A heart attack usually occurs when a blood vessel in the heart narrows or becomes completely blocked. As a result, the heart muscle receives too little oxygen, which can lead to permanent damage. Typical symptoms include severe chest pain, shortness of breath, or a feeling of tightness in the chest. A cerebral infarction—often referred to as an ischemic stroke—is caused by a disruption in blood flow to the brain. Those affected may suddenly experience paralysis, speech difficulties, dizziness, or vision loss. Both conditions are considered medical emergencies and require rapid specialized treatment to prevent permanent damage or life-threatening consequences as much as possible.

“Previous studies have linked irregular sleep habits to risks for heart health, but this is the first time we have separately examined variations in bedtime, wake-up time, and the midpoint of the sleep phase—as well as their independent associations with major cardiac events,” says postdoctoral researcher Laura Nauha from the University of Oulu.

Tracking Sleep Habits and Long-Term Health

The University of Oulu study followed a total of 3,231 people from northern Finland over an exceptionally long period. All participants were part of a birth cohort from 1966 and were approximately 46 years old at the time of sleep measurement. Unlike in many earlier studies, sleep habits were not only recorded using subjective questionnaires but also objectively measured using activity monitors. These devices tracked, over the course of a week, when participants went to bed, when they woke up, and how much their sleep times varied from day to day. The researchers then tracked the participants’ health status for more than ten years using national health registries. This allowed them to determine whether people with particularly irregular sleep schedules were more likely to develop serious cardiovascular diseases such as heart attacks or strokes later in life. Such long-term observational studies are considered particularly valuable because they can reveal links between everyday behaviors and future health risks.

According to the researchers, highly fluctuating bedtimes in particular could put a strain on the body’s internal biological clock. This so-called circadian rhythm controls numerous vital processes, including blood pressure, heart rate, hormone balance, metabolism, and inflammatory responses. If bedtime constantly varies, the body must repeatedly readjust, which could lead to increased strain on the cardiovascular system over the long term. The study’s findings therefore suggest that it is not just the duration of sleep that matters, but also the regularity of the sleep rhythm. People who go to sleep at roughly the same time every day may thereby support the stability of their biological processes and promote their heart health in the long term. At the same time, the researchers emphasize that this is a statistical correlation and not definitive proof of cause and effect. Nevertheless, a regular sleep rhythm is considered a lifestyle factor that many people can consciously influence.

Too Much – or too Little – Sleep Increases the Risk of Heart Attack

Previous research from the University of Colorado Boulder has already shown that too little—or too much—sleep can increase the risk of heart attack. The study also found that for individuals with a high genetic risk of heart attack, getting 6 to 9 hours of sleep per night can offset this risk. For the study, researchers from Massachusetts General Hospital and the University of Manchester analyzed the genetic data, self-reported sleep habits, and medical records of 461,000 participants in the UK Biobank aged 40 to 69 who had never suffered a heart attack, and then followed them over a period of seven years. Compared to those who slept 6 to 9 hours per night, those who slept less than six hours were 20 percent more likely to suffer a heart attack during the study period. For those who slept more than nine hours, the likelihood was 34 percent higher. When the researchers focused only on individuals with a genetic predisposition to heart disease, they found that sleeping six to nine hours per night reduced their risk of a heart attack by 18 percent.

For the study, the researchers used the extensive UK Biobank dataset and combined observational and genetic research to examine the question from a different angle. After accounting for 30 other factors—including body composition, physical activity, socioeconomic status, and mental health—they found that sleep duration itself influenced heart attack risk independently of these other factors. The further sleep duration deviated from the 6- to 9-hour range, the greater the increase in risk. For example, people who slept five hours per night had a 52 percent higher risk of heart attack than those who slept 7 to 8 hours, while those who slept 10 hours per night were twice as likely to have a heart attack.

Using a method called “Mendelian randomization,” the researchers then examined the participants’ genetic profiles to determine whether those genetically predisposed to short sleep had a higher likelihood of heart attacks. Twenty-seven genetic variants were associated with short sleep. They observed similar patterns and concluded that genetically determined short sleep duration is a risk factor for heart attacks.

The study did not examine the mechanism by which short or long sleep might increase the risk of heart attack, but previous studies have suggested some explanations. Too little sleep can impair the lining of the arteries (the endothelium), influence the formation of inflammatory cells in the bone marrow, and also lead to an unhealthy diet and poor eating habits (which in turn can affect weight and thus heart health). Too much sleep can also increase inflammation in the body, which is likewise associated with cardiovascular disease. The authors hope that the study will raise awareness among doctors, health authorities, and the general public about the benefits of sleep for heart health.