People who regularly exercised early in the morning were significantly less likely to have coronary artery disease, high blood pressure, Type 2 diabetes, or obesity compared with those who exercised later in the day, according to research presented at the American College of Cardiology's Annual Scientific Session (ACC.26) on Mar. 19.
The study is important because it suggests that not only the amount but also the timing of physical activity may influence cardiometabolic health. Cardiometabolic diseases such as high blood pressure, diabetes, and obesity are known risk factors for heart disease, which remains a leading cause of death worldwide.
Researchers analyzed health records and Fitbit-derived heart rate data from over 14,000 participants in the All of Us national study. They tracked minute-level heart rate data over a year to identify periods when participants had an elevated heart rate for at least 15 consecutive minutes. This approach focused on the body's response to exercise rather than specific activities like walking or gym workouts.
Participants were grouped based on when they typically exercised during the day. The analysis found that those who frequently exercised in the morning were 31% less likely to have coronary artery disease, 18% less likely to have high blood pressure, 21% less likely to have hyperlipidemia, 30% less likely to have Type 2 diabetes, and 35% less likely to be obese compared with those exercising later in the day. These associations held even after accounting for total daily physical activity and other factors such as age, sex, income level, sleep duration, alcohol use, and smoking status. Exercise between 7:00 and 8:00 a.m. was linked with the lowest odds of coronary artery disease.
Researchers said this is one of the first large studies using long-term wearable device data to assess both exercise amount and timing. "In the past, researchers have mainly looked at how much physical activity to do, the number of minutes or the intensity of physical activity," Patel said. "Now with 1 in 3 Americans having a wearable device, we're gaining the ability to look at exercise at the minute-by-minute level, and that opens a lot of doors in terms of new analyses."
The findings show only an association between early morning exercise and improved health markers; they do not prove causation. Patel said biological factors like hormones or genetics could play a role in these associations. Behavioral factors might also contribute—for example, people who exercise early may make healthier choices throughout their day or already prioritize healthy habits overall. Researchers noted that further studies are needed to clarify these relationships and inform future recommendations.
