Millions of Americans make time in their busy schedule to exercise daily. But only 23% of adults ages 18 and older meet the recommended guidelines for aerobic and muscle-strengthening activity. The biggest hurdle for most people: not having enough time. Au contraire, says a 2019 study by the CDC and Rand. The survey of more than 30,000 participants found that Americans have an average of more than five hours of free time per day.
Whether you’re considering starting an exercise regimen or a more experienced athlete, one of the biggest questions I hear is, “When is the best time to exercise?” Most people are fairly disciplined and protective when they exercise. Choosing to exercise in the morning or evening is often a product of a work schedule or childcare responsibilities. Or just whether you are a ‘morning person’ or a ‘night owl’.
But is there any science to support exercise in the morning versus evening? A recent study in Frontiers in Physiology shed some light.
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Does the early bird get the movement worm?
This was a relatively small study from Skidmore University that collected data from 27 women and 20 men who were already highly active on a regular exercise regimen. The participants were followed for 12 weeks. They did one of four exercise routines for an hour at a time four times a week — stretching, resistance training, interval sprints, or endurance training. One group did the routine between 6.30 am and 8.30 am, and the other was between 6 pm and 8 pm.
For the group that exercised in the morning:
Women had 7% more loss of belly fat, greater blood pressure drop, and more leg strength
For the group that exercised in the evening:
Women had greater increases in upper body strength, power, and endurance and improvement in mood Men had better heart health, metabolic health, and emotional well-being
Deeper dive into other research
Previous studies on the time-dependent effects of exercise were inconsistent across the board with the results of this new study. In contrast, a small 2019 study found that men also lost more weight when they exercised in the morning. But multiple previous studies support the current study’s finding of improved metabolic health in men who exercised in the afternoon, including better insulin sensitivity and blood glucose control.
An international consortium of researchers conducted a fascinating study in January 2022 on the molecular changes in the cells of multiple organs in mice to try to quantify at the most basic cellular level what happens when morning versus evening. It is being trained. The molecular profiles in mice showed a greater reliance on fat to fuel morning workouts and a greater reliance on glucose to fuel afternoon workouts. While some may argue that we cannot extrapolate data from mice to humans, the cellular processes are similar at the molecular level.
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Additional factors believed to play a role include sleep quality and hormones.
The role of sleep
One possible explanation is that women spend longer in the deep sleep phase and therefore tend to be more alert and ready to exercise earlier in the morning. But plenty of men also prefer to exercise in the morning. This brings us to one of the biggest myths about sleep and exercise; that exercising too late in the evening or just before bed will lead to reduced sleep quality. Again, it depends. Exercising late in the day may not affect those self-described night owls. Most importantly, a meta-analysis identified 29 studies that showed exercise improved sleep quality or duration.
Don’t count hormones.
Levels of cortisol, the stress hormone, vary higher for both men and women in the morning. This can lead to men and women who prefer cardio-style workouts earlier in the morning having to “burn off” the stress. However, cortisol can have an inhibitory or catabolic effect on muscle building. So men and women whose goal is strength training can see more benefits from evening workouts.
Also worth noting is that the latest study found that macronutrient consumption played no role. Study participants were also required to follow the same eating regimen of five meals per day at the same time for 12 weeks.
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Bottom Line: This was a small study, and we still don’t know much about the time-dependent benefits of exercise. This study showed a clear benefit from a morning workout for women whose goal is to burn fat. It adds to the evidence that the metabolic benefits are higher for men who exercise in the evening. And I don’t think we can ignore the catabolic effects of cortisol; for men and women whose goal is to build strength, evening workouts may be preferable.
Whether you exercise in the morning or the evening, the most important point is that you are exercising, and you are sure to reap its many benefits. If you feel better mentally and enjoy exercising first thing in the morning, stick with it! If you have a specific goal in mind, consider the results of the studies when choosing your time of day to exercise.
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Michael Daignault, MD, is a board-certified ER physician in Los Angeles. He studied Global Health at Georgetown University and received a medical degree from Ben-Gurion University. He completed his residency training in emergency medicine at Lincoln Medical Center in the South Bronx. He is also a former United States Peace Corps volunteer. Find him on Instagram @dr.daignault.
