People who are more active at night may face a higher risk of heart disease than those who are active earlier in the day, especially women, according to a large new study published in the Journal of the American Heart Association.
Researchers analyzed health data from more than 300,000 middle-aged and older adults enrolled in the UK Biobank to examine how chronotype—a person’s natural preference for sleeping and waking—affects cardiovascular health. The average age of participants was 57.
Night owls
About 8% of participants identified as “definitely evening people,” often going to bed very late and becoming most active later in the day. Around 24% described themselves as “definitely morning people,” with earlier bedtimes and higher activity levels in the morning. The majority, about 67%, fell into an intermediate category, with no strong preference for either.
Using the American Heart Association’s Life’s Essential 8—a set of metrics that include diet, physical activity, smoking, sleep quality, weight, cholesterol, blood sugar, and blood pressure—the researchers assessed overall heart health.
The study found that evening types had a 79% higher prevalence of poor cardiovascular health compared with people in the intermediate group. Over a median follow-up of nearly 14 years, night owls also had a 16% higher risk of heart attack or stroke.
The link between an evening chronotype and poor heart health was stronger among women than men, the researchers said.
‘Circadian misalignment’
Much of the increased risk was driven by unhealthy behaviors and risk factors, particularly smoking and inadequate or irregular sleep. In contrast, people who identified as morning types had a 5% lower prevalence of poor heart health compared with those without a strong chronotype preference.
“Evening people often experience circadian misalignment, where their internal body clock does not match the natural day–night cycle or their daily schedules,” said lead author Sina Kianersi, a research fellow at Brigham and Women’s Hospital and Harvard Medical School. This misalignment may contribute to poorer sleep, diet, and other behaviors that affect heart health.
Still, experts say the findings are not a verdict against night owls. Kristen Knutson, who chaired the American Heart Association’s 2025 scientific statement on circadian health and was not involved in the study, said the elevated risk is partly due to modifiable habits.
“These results show that evening types have options to improve their cardiovascular health,” Knutson said, pointing to smoking cessation, better sleep routines, and healthier lifestyles. She added that medical treatments and interventions may be more effective if they take a person’s chronotype into account.
The researchers noted limitations to the study, including the fact that most participants were white and generally healthier than the wider population. Chronotype was also self-reported and measured only once.
Still, the findings add to growing evidence that when people are active—and how well that timing aligns with their internal body clock—may matter as much as how active they are when it comes to heart health.
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