Wake up and work out: Early exercise may slash heart disease risk

The research is based on health records and Fitbit-derived heart rate data from over 14,000 people. Although it is unclear whether the relationship between exercise timing and cardiometabolic health is causal or mediated by other factors, researchers said the findings could inform approaches for counseling patients on physical activity based on a more granular look at exercise behaviors than has been possible before.

“Any exercise is going to be better than no exercise, but we tried to identify an additional dimension relating to the timing of exercise,” said Prem Patel, a medical student at the University of Massachusetts Chan Medical School and the study’s lead author. “If you can exercise in the morning, it seems to be linked with better rates of cardiometabolic disease.”

Cardiometabolic disease markers such as high blood pressure, diabetes and obesity are associated with an increased risk of heart disease, the leading cause of death worldwide. Getting regular physical activity is known to reduce a person’s risk of these markers as well as serious cardiac events.

Researchers analyzed data from 14,489 individuals participating in All of Us, a large national study. Minute-level heart rate data from Fitbit devices were analyzed over the course of a year. To track bursts of physical activity, the researchers identified periods when participants had an elevated heart rate for 15 consecutive minutes or more. The methodology differs from other studies since it is based on the body’s response to exercise—elevated heart rate—rather than tracking particular activities such as walking to work, doing household chores or working out at the gym.

Researchers assessed each participant’s exercise in 15-minute intervals throughout the day and grouped participants into categories based on their timing of exercise. Based on health records, researchers analyzed associations between exercise timing and high blood pressure, diabetes, obesity and hyperlipidemia (elevated LDL-C cholesterol or triglycerides). They also assessed rates of cardiovascular outcomes such as coronary artery disease and atrial fibrillation and accounted for other risk factors relevant to cardiovascular health such as age, sex, income level, total activity level, sleep duration, alcohol use and smoking status.

Compared to those who exercised later in the day, 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 have obesity. These associations were independent of the total daily amount of physical activity. Exercise between 7:00 and 8:00 a.m. was associated with the lowest odds of coronary artery disease.

Researchers said that the study offers a more holistic view of exercise than previous research and suggest that exercise timing may represent a previously underappreciated aspect of cardiometabolic health. Most studies have historically assessed total physical activity levels or tracked exercise timing for short periods; this is the first large study to assess exercise amount and timing based on long-term data from wearable devices.

“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 an association only and do not indicate whether early exercise habits cause improvements in health markers, researchers said. Patel said that biological factors such as hormones, sleep or genetics may play a role in the associations they observed. Behavioral and psychological factors also could be involved; for example, exercising early in the day could lead to higher energy levels and healthier food choices throughout the day, or it could be more common among people who prioritize healthy habits in general. Further research could help to determine the role of such factors and help inform exercise recommendations, the authors said.