“This study suggests that when you exercise may matter, not just how much you exercise,” says senior study author Prashant Rao, MBBS, a sports cardiologist and physician-scientist at Beth Israel Deaconess Medical Center and Harvard Medical School, both in Boston.
The research, which will be presented at the American College of Cardiology’s Annual Scientific Session later this month, relied on minute-level heart rate data from nearly 15,000 adults. Dr. Rao says this allowed his team to capture long-term, real-world exercise data with much more detail and accuracy.
Early Morning Workouts Are Linked to Better Cardiometabolic Health
The study analyzed health records and Fitbit heart rate data collected over a year. Researchers identified periods when participants had an elevated heart rate for 15 minutes or more to track physical activity. Then they grouped participants into categories based on the time of day exercise occurred.
Researchers compared these timing groups with health data including rates of high blood pressure, obesity, high cholesterol, atrial fibrillation, coronary artery disease, and other cardiovascular health problems. They adjusted the findings to account for differences in age, sex, income level, total activity level, sleep, alcohol use, and smoking status.
The results showed that compared with adults who exercised later in the day, those who worked out in the morning were:
These risk reductions were independent of how much exercise people actually got. Adults who exercised between 7 and 8 a.m. had the lowest odds of coronary artery disease.
Aubrey Grant, MD, a sports cardiologist at MedStar Health in Washington, D.C., who wasn’t involved with the research, says the results suggest that “timing may be a previously underappreciated lever in cardiometabolic risk reduction.”
He adds that wearable technology is giving scientists more granular exercise data, which “opens a more nuanced conversation about how physical activity interacts with the body’s daily rhythms.”
Exercise Timing Is a ‘New Frontier’
A main limitation of the research is that it’s an observational study, “so we can’t establish causality,” Rao says. Because participants weren’t randomized to exercise at different times, the results only show a link, not that earlier exercise directly leads to the observed health benefits.
“While we did our best to adjust for confounders [factors that could muddy the results], timing may still reflect differences in work schedules, socioeconomic factors, or lifestyle patterns that aren’t potentially fully captured in our analyses,” he says.
The link between exercise timing and health is a “new frontier in exercise science,” says Dr. Grant. For decades, research has focused on “how much and how hard” people worked out — variables that still matter, he says.
Why Exercise Timing May Lower Cardiometabolic Risk
“The honest answer is that we do not fully know yet” why this link is appearing, Grant says.
“Morning exercise may align better with circadian physiology,” or how bodily functions naturally correspond to the time of day, he says. “Cortisol peaks early in the day and can prime the body for physical exertion, potentially enhancing metabolic efficiency,” Grant adds.
Exercise is also a natural stimulant. When you work out early, it revs up your bodily systems and energizes you for the day, says Andrew Freeman, MD, director of cardiovascular prevention and wellness at National Jewish Health in Denver. Dr. Freeman was not involved with the new study.
People who work out earlier may also have healthier lifestyle habits, overall, says Alex Rothstein, EdD, an assistant professor of exercise science at the New York Institute of Technology in Old Westbury.
“If you work out earlier in the day, you tend to have fewer excuses to not work out,” and you may exercise more consistently, says Dr. Rothstein, who was not involved with the new research.
Should You Start Exercising in the Morning?
Freeman recommends getting 30 minutes a day of “breathless physical activity,” including a combination of cardio and strength training. But if you prefer to work out in the afternoons or have responsibilities preventing you from exercising in the morning, don’t worry.
“It does not mean people should feel guilty about evening workouts,” Grant says. “It means that when we counsel patients on optimizing their health, exercise timing is now a legitimate part of that conversation, alongside sleep, nutrition, and stress management.”
If you have flexibility, though, Rothstein suggests exercising in the morning, as it’s less likely that something will interfere with your workout later on, and it may offer extra heart health benefits.
“The most important message is still: Exercise consistently, regardless of timing,” Rao says. However, “Timing may represent a simple, low-cost way to potentially optimize health.”