Should you exercise first thing in the morning or at night? Here's what research says.

Millions of Americans carve time out of their busy schedule to exercise daily. But only 23% of adults aged 18+ meet the recommended guidelines for both aerobic and muscle-strengthening activity. The biggest hurdle for most people: Not having enough time. Au contraire, says a 2019 study from the CDC and Rand. Surveying more than 30,000 participants, the study found that Americans have an average of more than five hours of free time per day.

Whether you're considering starting a workout regimen or a more seasoned athlete, one of the biggest questions I hear is, “When is the best time to exercise?” Most people are fairly regimented and protective of when they exercise. Choosing to exercise in the morning or evening is often a product of a work schedule or childcare responsibilities. Or simply whether one is a “morning person” or a “night owl.”

But is there any science to support working out in the morning versus working out in the evening? A recent research study in Frontiers in Physiology sheds some light.

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The early bird gets the exercise worm?

This was a relatively small study from Skidmore University that collected data from 27 women and 20 men who were already highly active with a regular exercise regimen. Participants were followed over 12 weeks. They did one of four different exercise routines – stretching, resistance training, interval sprints or endurance training –four times a week for one hour each time. One group did the routine between 6:30 and 8:30 a.m. and the other group between 6 and 8 p.m.

For the group who exercised in the morning:

• Women had a 7% greater loss of belly fat, larger reduction in blood pressure and greater leg strength

For the group who exercised in the evening:

• Women had a greater increase in upper body strength, power and endurance and mood improvement

• Men had improved heart health, metabolic health and emotional well-being

• Men also had a greater weight loss and reduction in blood pressure

Deeper dive into other research

Previous studies that looked into the time-dependent effects of exercise were not consistent across the board with the results of this new study. In contrast, a small 2019 study found that men also had greater weight loss if they exercised in the morning. But multiple previous studies do support the current study 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 in January 2022 did a fascinating research study that looked at the molecular changes occurring in the cells of multiple organs in mice to try to quantify at the most basic cellular level what happens when exercise is done in the morning versus the evening. The molecular profiles in mice showed a greater reliance on fat to fuel morning exercise and a greater reliance on glucose to fuel afternoon workouts. While some might argue we cannot extrapolate data on mice to humans, the cellular processes at molecular levels are similar.

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Additional factors that have been hypothesized to play a role include sleep quality and hormones.

The role of sleep

One possible explanation is that women tend to spend longer in the deep sleep stage and therefore tend to be more alert and ready to exercise earlier in the morning. But there are plenty of men who prefer to exercise in the morning as well. This brings us to one of the biggest myths of sleep and exercise; that working out too late in the evening or close to bedtime will lead to reduced sleep quality. Again, it depends. Exercising late in the day might not affect those self-described night owls. And most importantly, a meta-analysis identified 29 studies which demonstrated that exercise improved sleep quality or duration.

Don't count out hormones

Levels of cortisol, the stress hormone, range higher for both men and women in the morning. This might generate a need to "burn off" stress for both men and women who prefer to do cardio-type workouts earlier in the morning. Cortisol, however, can have an inhibitory, or catabolic effect, on muscle building. So men and women whose goal is strength training might see greater benefits with evening workouts.

Also worth mentioning is that the newest study found that macronutrient consumption did not play a role. Participants in the study were also required to keep to the same exact eating regimen of four meals a day at the same times for 12 weeks.

The 'X' factor: You do you

Bottom line: This was a small study and there’s still a lot we don’t know about the time-dependent benefits of exercise. It adds to the body of evidence that metabolic benefits are higher for men who exercise in the evening. For women whose goal is to burn fat, this study did demonstrate a clear benefit to a morning workout. And I think we can’t ignore the catabolic effects of cortisol; for men and women whose goal is building strength, an evening workout might be preferred.

Whether you exercise in the morning or evening, the key point is that you are exercising and certainly reaping its many benefits. If you feel mentally better and enjoy working out first thing in the morning, then stick with that! If you have a specific goal in mind, then consider the results of the studies in choosing your time of day to work out.

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Michael Daignault, MD, is a board-certified ER doctor in Los Angeles. He studied Global Health at Georgetown University and has 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

This article originally appeared on USA TODAY: Exercise in the morning or night? Depends on your goal, new study says