Being awake and active late may be linked to worse cardiovascular outcomes, according to research that followed hundreds of thousands of people in the UK Biobank for more than a decade. The study, reported in the Journal of the American Heart Association, found that people classified as “night owls” had higher risk for a first heart attack or stroke and also had worse overall cardiovascular health based on whether they met a set of eight American Heart Association heart-health factors.
Lead author Sina Kianersi of Brigham and Women’s Hospital and Harvard Medical School said the result is not simply that night owls are “doomed,” but that the timing of their internal body clock can clash with the everyday schedules most people follow. “It is not like, that, night owls are doomed,” Kianersi said. “The challenge is the mismatch between your internal clock and typical daily schedules ” that makes it harder to follow heart-healthy behaviors.
In describing why circadian timing might matter for heart health, the researchers pointed to the body’s master biological clock, which roughly follows a 24-hour schedule and helps coordinate systems beyond sleepiness and alertness. The circadian rhythm influences processes that include heart rate, blood pressure, stress hormones and metabolism, making timing relevant to cardiovascular risk.
The study analyzed information from more than 300,000 middle-age and older adults in the UK Biobank, a health database that includes self-reported sleep-wake preferences. About 8% of participants classified themselves as night owls—people who reported being more physically and mentally active in the late afternoon or evening and staying up past most people’s bedtime—while about a quarter identified as early birds, most productive during daylight hours and going to bed earlier; the remaining participants fell in between.
Over 14 years of follow-up, the night-owl group had a 16% higher risk of a first heart attack or stroke than the average population, the researchers reported. The study also found that the night owls, especially women, had worse overall cardiovascular health when measured against the American Heart Association’s eight key factors, which include being more physically active, avoiding tobacco, getting enough sleep, and eating a healthy diet, along with controlling blood pressure, cholesterol, blood sugar and weight.
Kianersi and his team said unhealthy behaviors—including smoking, insufficient sleep and poor diet—appear to play a main role in the associations they observed. Even where night owls may already be active later, the issue may be how hard it is to align heart-healthy routines with typical morning-to-evening schedules, particularly for work that starts early, the study’s authors said.
Kristen Knutson of Northwestern University, who led recent American Heart Association guidance on circadian rhythms but was not involved in the new research, said the misalignment can ripple beyond sleep. “It comes down to the problem of a night owl trying to live in a morning person’s world. They’re getting up early for work because that’s when their job starts but it may not align with their internal rhythm,” Knutson said.
Knutson added that circadian timing can also affect metabolism, including how the body handles food. She said metabolism fluctuates throughout the day as the body produces insulin to turn food into energy, meaning it might be harder for a night owl to handle a high-calorie breakfast eaten very early during what would still be their biological night. She also said that when people are out late at night, it can be harder to find healthy food choices.
For sleep-related steps, Knutson and Kianersi said that even if someone can’t reach the ideal of at least seven hours of sleep, keeping a regular bedtime and wake time may help with better alignment. And while the study could not examine what night owls do when the rest of the world is asleep, Kianersi said one of the best actions to protect heart health—both for night owls and for everyone—is to quit smoking. “Focus on the basics, not perfection,” he said, describing advice he said applies to all sleep schedules.