Starting your day with a workout may make for an extra good morning if you’re trying to manage your weight. Exercising in the a.m. was associated with a lower average body mass index (BMI) and waist circumference compared with getting physical at other times of the day, according to an analysis published this week in the journal Obesity. Both BMI and waist size are standard measures used to determine if a person is overweight or obese, according to the National Institutes of Health (NIH).
Researchers, led by Tongyu Ma, PhD, research assistant professor in the department of rehabilitation services at the Hong Kong Polytechnic University, reviewed two years of health data from 5,285 American adults who participated in the Center for Disease Control and Prevention’s (CDC) National Health and Nutrition Examination Survey (NHANES). Participants wore accelerometers, activity trackers that could determine whether they were engaging in moderate to vigorous physical activity, which was the main focus in this study.
What the study authors discovered was that people who primarily engaged in such activity early in the day (from 7 a.m. to 9 a.m.) were more likely to have a lower BMI and waist circumference compared with those who participated in similarly rigorous exercise midday (between 9 a.m. and 4 p.m.) or in the evening (between 5 p.m. and 8 p.m.). The group that exercised in the morning also reported consuming fewer daily calories per kilogram of body weight and having a healthier diet overall compared with the other two groups.
Interestingly, though, the morning exercise group tended to be older, on average, than the two other groups (average age was 60 in that group, compared with 50 in the midday workout group and 46 in the evening group), and more sedentary, spending a greater number of hours sitting throughout the day.
One other distinction the analysis found: Getting the 150 minutes per week of moderate to vigorous physical activity recommended by the Physical Activity Guidelines for Americans was associated with a lower BMI and waist size among all groups, but only those who exercised in the morning had lower numbers corresponding to the increased duration of their workouts. In groups that exercised at other times of the day, there was no link between additional minutes of exercise and lower BMI or waist size. Although the investigation did not explore why this was the case, Dr. Ma suggested that it may be due to different diet and eating habits among people who get more vigorous physical activity.
As with any observational study, there are limitations to what these findings suggest. Because measurements were only taken once before activity tracking began, scientists could not determine if early morning exercise directly caused lower measures of BMI and waist size.
“It is important to note that the data in this study are just a snapshot of one moment in time,” says Becca Krukowski, PhD, a clinical psychologist with expertise in behavioral weight management interventions at the University of Virginia in Charlottesville, who was not involved in the study. “These data do not look at patterns over time, so we can’t know the direction of the relationship between physical activity and BMI.”
Ma and the other authors of this recent study acknowledge that, while moderate to vigorous physical activity is known to be inversely associated with obesity, additional prospective studies and randomized clinical trials are needed to confirm the benefits of exercising at a specific time. Some past research, which was highlighted in the current study, indicates that morning physical activity after an overnight fast may be more effective in promoting fat oxidation, the process by which the body breaks down fats, compared with evening exercise.
“Morning exercise is a promising tool, yet there hasn’t been a consensus,” says Ma. In fact, one randomized controlled study of 100 people with overweight or obesity found that both morning and evening workouts appeared beneficial for weight loss. The results of that study, which were published in Obesity in 2022, indicate that there may not be an optimal time to exercise, but it was a small study and additional research is still needed to draw any larger conclusions.
“Overall, this study area is important because we are always looking for ways to optimize workout time,” says Lance Walker, PT, director of Oklahoma State University’s Human Performance and Nutrition Research Institute in Stillwater, who was not involved in the study. Whether you are naturally an early bird or a night owl, however, these study results should not discourage you from exercising when it is most convenient or feels best. “Some people may think: ‘If I can't work out in the morning, I won't because it's not going to be effective,’” Walker says. “That’s not the message. There’s not a one-size-fits-all time to exercise. You have to identify what is your optimal time to exercise.”
There are lots of factors that come into play when deciding when to work out. For some people, exercising later in the day could have a negative effect on sleep quality, and poor sleep may heighten the risk of weight gain and obesity. Others may find that exercising before their day gets started helps them stick to a routine without interruptions. “Morning exercise may be more consistent because it occurs before e-mails, phone calls, or meetings that might distract you,” says Krukowski.
“Given the mixed findings about the timing of exercise in the literature thus far — as well as the overwhelming body of evidence indicating how important physical activity is for overall health,” she says, “I feel confident in recommending exercising at the time that best fits your schedule.”