Irregular sleep and the heart: a study of 1,992 adults

The MESA study (Multi-Ethnic Study of Atherosclerosis), published in the Journal of the American College of Cardiology in 2020, looked at how regular sleep is, not only at how much. According to the authors, irregular sleep duration and timing may be new cardiovascular risk factors, independent of the traditional ones and of how much people sleep.

The study included 1,992 adults without cardiovascular disease. For 7 days they wore a wrist actigraph, a device that records sleep from movement. The researchers measured how much sleep duration and the time of falling asleep varied from one night to the next. They then followed the participants for a median of 4.9 years: in that period 111 people had a cardiovascular event.

The cardiovascular system follows strong circadian rhythms, tied to the alternation of day and night. Irregular sleep schedules disrupt those rhythms, perhaps more mildly than shift work, but they are far more common in the population. That is the starting hypothesis of the study.

The study's numbers

Compared with people whose sleep duration varied by up to 60 minutes, those above 120 minutes had roughly double the risk of cardiovascular events (hazard ratio 2.14). Between 91 and 120 minutes the increase (1.59) was not statistically significant.

For the time of falling asleep the comparison is between up to 30 minutes of variation and more than 90: here too the risk was roughly double (2.11). The results account for traditional cardiovascular risk factors and average sleep duration, and remain similar when shift workers are excluded.

Another study, published in Nature Medicine in 2024, looked instead at sleep stages: more REM sleep and more deep sleep were associated with lower odds of atrial fibrillation. Duration, regularity and stages are different aspects of the same sleep, and research studies them one at a time.

The Digital Twin and this research

The Digital Twin lines up the data your devices write to your phone, such as steps, heart rate and sleep, and shows you how they change over time. It does not calculate your cardiovascular risk and it does not diagnose: this research describes groups of people, not you. What the regularity of your sleep means is something you assess with your doctor.