People Who Workout in the Morning Are Less Likely to Be Obese

Professor Ma and his team’s research looked at 5,285 individuals who participated in the Nutrition Examination Survey (NHANES) between 2003 and 2006. The NHANES is a survey conducted by the Centers for Disease Control and Prevention (CDC) and is intended to serve as representative data for the population of the United States. Using self-reported information for things like, age, sex, and diet, as well as data from an accelerometer, a device worn on the hip that’s used to track activity, Ma wanted to see if there was any correlation between when individuals exercised and two key indicators of obesity: BMI and waist circumference.

Individuals were placed into three groups based on when they exercised during the day: morning, afternoon, and evening. Researchers found that individuals who worked out in the morning had a lower average BMI (27.4) compared to those who worked out in the afternoon (28.4) and evening (28.2). Waist circumference showed a similar association; the morning exercise group’s average waist was 95.9 centimeters (37.7 inches,) compared to 97.9 (38.5 inches) for the afternoon group, and 97.3 (38.3 inches) for the evening group.

Moreover, using the above indicators, researchers found a downward linear trend between minutes of exercise per week and obesity — meaning the more exercise they did per week — the lower the average BMI. A weaker trend was observed in the afternoon, midday or evening groups.

Interestingly, the morning workout group exercised less and engaged in more sedentary time than the other groups, but still maintained better BMI and waist size.

And the trend for the morning workout group continued even after controlling for potentially confounding factors like age, sex, ethnicity, tobacco use, and diet.

“The evidence implies that doing the right thing at the right time could be important for maintaining a healthy metabolism and a healthy body weight,” says the study.

Researchers note more study is needed and that there are several caveats for this study

Since it is a cross-sectional observational study, researchers can only observe trends. They can’t know for certain the cause of that trend.

Dr. Michael Fredericson, FACSM, CAQSM Director of the PM&R Sports Medicine and Co-Director of the Stanford Center on Longevity at the Stanford University School of Medicine, said the way the study was conducted it was unclear if the people who exercised in the morning were “systematically different from those who exercise at other times in ways not measured in this study.”

“For example, people who exercise regularly in the morning could have more predictable schedules, such as being less likely to be shift workers or less likely to have caregiving responsibilities that impede morning exercise,” Fredericson said.

The researchers don’t claim to fully understand the mechanism or reason why exercising in the morning appears to be more effective for obesity than working out at other times of the day.

The key to that question likely has to do with circadian rhythms, your body’s natural sleep-wake cycle that encompasses a myriad of hormonal and other biological processes.

“Many physiological processes display circadian rhythms, including the metabolisms that are closely associated with obesity,” said Ma. “There is general evidence that timing of exercise is important. However, more studies are needed to explicitly explain the significance of circadian oscillations regarding weight and weight loss,” he said.

Fredericson said the new study bolsters past research on the benefits of a morning workout.

“Studies have shown that morning exercise can help lower blood pressure, reduce stress, and decrease anxiety,” Fredericson said. “Additionally, working out in the morning helps to improve cognitive function by stimulating brain activity. It enhances alertness and concentration, increasing productivity throughout your day. Morning exercise, especially outdoors, also gives us morning light, which is critical to regulating our circadian rhythms.

Hormones linked with weight loss and obesity, such as insulin and cortisol, are closely tied to circadian rhythms. The presence and amount of these hormones oscillate as part of these rhythms and can affect things like sleepiness, hunger, fat burning (lipolysis), and fat storage (lipogenesis). Therefore, just like how the timing of when you eat can affect weight gain, so too can the timing of your exercise appear to affect weight loss.

“Research suggests that an eating schedule misaligned with one’s circadian rhythm can increase a person’s risk of becoming obese or developing metabolic problems like insulin resistance,” Fredericson said. “Habits like eating late at night or skipping breakfast are considered misaligned eating.”

Exercising in the morning also means that you are likely in a “fasted state,” assuming you don’t eat before your workout, which has been linked to greater “lipid mobilization and oxidation,” AKA fat burning.

Previous research has found that morning workouts are better for burning fat. In an animal study published in February 2023, researchers found that when mice did physical activity during an early active phase (mice are nocturnal, so morning isn’t the right word) they expressed a higher metabolic rate.

Another study, cited by Ma and his team, known as the Midwest Exercise Trial 2, found that individuals had a greater reduction in body mass and fat mass when they attended 50% or more of their exercise sessions in the morning, compared with those who attended 50% or more of their sessions in the late afternoon and evening.

Working out in the morning appears more effective for obesity than working out at other times of the day, although the reason why isn’t totally clear. However, study authors say more research is needed.

Researchers believe that circadian rhythms and their associated hormones likely play a role in the effectiveness of exercise and fat loss.