People who typically exercise in the morning may have a lower risk of high blood pressure, type 2 diabetes, obesity, and other causes of heart disease than those who exercise later in the day, according to a new study.
“This study suggests that it may not be how much you exercise, but when you exercise that matters,” says lead study author Prashant Rao, MBBS, a sports cardiologist and physician-scientist at Beth Israel Deaconess Medical Center and Harvard Medical School in Boston.
The study, to be presented later this month at the American College of Cardiology’s annual scientific session, is based on minute-by-minute heart rate data from about 15,000 adults. Dr. Rao says this allows his team to collect long-term, real-world exercise data in greater detail and accuracy.
Early morning training is associated with improved cardiometabolic health
The study analyzed health records and Fitbit heart rate data collected over a one-year period. To track physical activity, researchers identified periods during which participants’ heart rates increased for 15 minutes or more. Participants were then divided into categories based on the time of day the exercise took place.
Researchers compared these timing groups to health data, including rates of high blood pressure, obesity, high cholesterol, atrial fibrillation, coronary artery disease, and other cardiovascular health problems. They adjusted their findings to account for differences in age, gender, income level, total activity level, sleep, alcohol consumption, and smoking status.
They found that compared to adults who exercised later in the day, adults who exercised in the morning were:
These risk reductions were independent of how much exercise people actually did. Adults who exercised between 7 a.m. and 8 a.m. had the lowest odds of coronary artery disease.
Aubrey Grant, M.D., a sports cardiologist at MedStar Health in Washington, D.C., who was not involved in the study, said the results “suggest that timing may be a previously underappreciated tool for reducing cardiometabolic risk.”
He added that wearable technology allows scientists to obtain more detailed movement data, which “allows for more nuanced conversations about how physical activity interacts with the body’s daily rhythms.”
The timing of exercise is a “new frontier”
The main limitation of the study is that it is an observational study, so it “can’t establish cause and effect,” Rao said. Because participants were not randomized to exercise at different times, the results only show an association, not that exercising earlier in the day directly led to the observed health benefits.
“Although we did our best to adjust for confounders (factors that could cloud the results), timing may still reflect differences in work schedules, socio-economic factors, or lifestyle patterns, which may not be fully captured in our analysis,” he says.
The relationship between exercise timing and health is “a new frontier in exercise science,” Dr. Grant says. For decades, research has focused on “how much and how hard” people exercised, but that remains an important variable, he says.
But more data on timing is starting to emerge. For example, another study published in January found that older adults who self-identified as night owls had worse cardiovascular health than adults who self-identified as morning owls.
Why exercise timing lowers cardiometabolic risk
“The honest answer is that we still don’t fully understand why this link appears,” Grant said.
“Morning exercise may better align with your circadian physiology,” he says. “Cortisol peaks early in the day and can prepare the body for exercise, potentially increasing metabolic efficiency,” Grant added.
Exercise is also a natural stimulant. “Exercising early can help activate your body’s systems and help you feel more energized for the day,” says Dr. Andrew Freeman, director of cardiovascular prevention and wellness at National Jewish Health in Denver. Dr. Freeman was not involved in the new study.
People who exercise early may have healthier lifestyles overall, says Dr. Alex Rothstein, assistant professor of exercise science at New York Institute of Technology Old Westbury.
“When you exercise earlier in the day, you have fewer excuses not to exercise and are more likely to exercise consistently,” says Dr. Rothstein, who was not involved in the new study.
Should I start exercising in the morning?
Freeman recommends 30 minutes a day of “breathless physical activity,” a combination of aerobic exercise and strength training. However, if you prefer to exercise in the afternoon or have responsibilities that prevent you from exercising in the morning, don’t worry.
“That doesn’t mean people should feel guilty about training at night,” Grant says. “This means that exercise timing is now a legitimate part of the conversation, alongside sleep, nutrition and stress management, as we counsel patients on optimizing their health.”
However, if you have the flexibility, Rothstein recommends exercising in the morning. That’s because it’s less likely that anything will interfere with your exercise later on, and it may provide additional benefits for your heart health.
“The most important message remains: exercise consistently, regardless of timing,” Rao says. However, “timing can be a simple, low-cost way to optimize health.”
