The body’s clock keeps metabolism, blood pressure, and heart rate running on schedule. But when an irregular sleep pattern disrupts this delicate ticking, what happens? A new study led by investigators from Brigham and Women’s Hospital measured participants’ sleep duration and timing, finding that over a five-year period, individuals who had the most irregular sleep experienced a two-fold increased risk of developing cardiovascular disease compared to those with the most regular sleep patterns. The team’s findings are published in The Journal of the American College of Cardiology.

“When we talk about interventions to prevent heart attacks and stroke, we focus on diet and exercise,” says lead author Tianyi Huang, ScD, of the Brigham’s Channing Division of Network Medicine. “Even when we talk about sleep, we tend to focus on duration—how many hours a person sleeps each night—but not on sleep irregularity and the impact of going to bed at different times or sleeping different amounts from night to night. Our study indicates that healthy sleep isn’t just about quantity but also about variability, and that this can have an important effect on heart health.”

Huang and colleagues examined data from the Multi-Ethnic Study of Atherosclerosis (MESA), a federally funded, prospective study that included 1,992 participants who did not have cardiovascular disease at the beginning of the study. MESA consisted of a diverse study population made up of white (38%), African American (28%), Hispanic (22%), and Chinese American (12%) participants, aged 45-84 years, from six field centers across the United States. Participants wore an activity tracker on their wrist for seven days that recorded their sleep, including bedtime, sleep duration, and wake time. They were then followed for an average of 4.9 years. During that time, 111 participants experienced cardiovascular events, including heart attack, stroke, and other adverse events.

The investigators divided participants into four groups ranging from those with the most irregular sleep patterns (two hours or more difference in sleep duration each night) and those with the most regular sleep patterns (less than an hour difference in sleep duration each night). They also compared those with the most consistent bedtimes (less than 30-minute difference each night) and most inconsistent bedtimes (90 minutes or more). The team found a two-fold increase in risk of cardiovascular events among those with the most irregular sleep patterns. The researchers estimate that for every 1,000 people following the most regular sleep pattern, only eight would have a cardiovascular event over one year; for every 1,000 people with the most irregular sleep patterns, 20 people would likely develop a cardiovascular event over one year.

While large for a study that uses wrist-worn activity trackers to measure sleep, the sample size for the study was modest and its follow-up time was relatively short, which meant that the researchers could not assess risk of individual adverse events such as a heart attack versus a stroke and the possibility of chance findings could not be fully excluded. If larger studies confirm these findings in the future, Huang would like to evaluate whether an intervention—such as, sleeping longer or more regularly—could decrease a person’s risk.

“Sleep regularity is a modifiable behavior. In the future, we’d like to explore whether changing one’s sleep patterns by going to bed consistently each night may reduce a person’s risk of future cardiovascular events,” Huang says.