Millions of Americans take time out of their busy schedules to exercise every day. But only 23% of adults over the age of 18 meet the recommended guidelines for aerobic and muscle-strengthening activity. The biggest obstacle for most people: not having enough time. On the contrarysays a 2019 study from the CDC and Rand. Surveying more than 30,000 participants, the study found that Americans average more than five hours of free time per day.
Whether you’re considering starting an exercise regimen or you’re a more experienced athlete, one of the biggest questions I hear is, “When is the best time to exercise?” Most people are quite regimented and protective when they exercise. The choice to exercise in the morning or at night is often a product of a work schedule or childcare responsibilities. Or just whether one is a “morning person” or a “night owl”.
But is there any science that supports exercising in the morning instead of at night? A recent research study in Frontiers in Physiology shed some light.
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Does the early bird get the exercise worm?
This was a relatively small study from Skidmore University that collected data from 27 women and 20 men who were already very active with a regular exercise regimen. The participants were followed for 12 weeks. They did one of four different exercise routines — stretching, resistance training, interval running or resistance training — four times a week for an 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 that exercised in the morning:
- Women had 7% greater loss of abdominal fat, greater reduction in blood pressure, and greater leg strength
For the group that exercised at night:
- Women had a greater increase in upper body strength, power and endurance and an improvement in mood
- The men had improved heart health, metabolic health, and emotional well-being
- The men also had greater weight loss and lower blood pressure.
Deeper dive into other investigations
Previous studies looking at time-dependent effects of exercise were not generally consistent 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 support the current study’s finding of better metabolic health in men who exercised in the afternoon, including better insulin sensitivity and blood glucose control.
In January 2022, an international consortium of researchers conducted a fascinating research study that looked at the molecular changes that occur in the cells of multiple organs in mice to try to quantify at the most basic cellular level what happens when you exercise in the morning and at night. . Molecular profiles in mice showed a greater reliance on fat to drive morning exercise and a greater reliance on glucose to drive afternoon workouts. While some might argue that we can’t extrapolate data from mice to humans, the cellular processes at the molecular level 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 more time in deep sleep and therefore tend to be more alert and ready to exercise earlier in the morning. But there are also many men who prefer to exercise in the morning. This brings us to one of the biggest myths about sleep and exercise; that exercising too late at night or close to bedtime will lead to reduced sleep quality. Again, it depends. Exercising late in the day might not affect self-described night owls. And most importantly, a meta-analysis identified 29 studies that showed exercise improved sleep quality or duration.
Don’t rule out hormones
Cortisol levels, the stress hormone, are higher for both men and women in the morning. This could create a need to “burn off” stress for both men and women who prefer to do cardio-type exercises earlier in the morning. Cortisol, however, can have an inhibitory or catabolic effect on muscle building. Therefore, men and women whose goal is strength training may see greater benefits with evening workouts.
It’s also worth mentioning that the most recent study found that macronutrient intake did not play a role. Study participants were also asked to maintain the exact same eating regimen of four meals a day at the same time for 12 weeks.
The ‘X’ Factor: You Do It
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 greater for men who exercise at night. For women whose goal is to burn fat, this study showed a clear benefit of exercising in the morning. And I think we can’t ignore the catabolic effects of cortisol; for men and women whose goal is to build strength, an evening workout may be preferred.
Whether you exercise in the morning or at night, the bottom line is that you’re exercising and no doubt reaping its many benefits. If you feel mentally better and like to exercise first thing in the morning, keep it up! If you have a specific goal in mind, consider the results of studies when choosing the time of day to exercise.
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Michael Daignault, MD, is a board-certified emergency room physician in Los Angeles. He studied Global Health at Georgetown University and has a Bachelor of Medicine from Ben-Gurion University. He completed his emergency medicine residency training at Lincoln Medical Center in the South Bronx. He is also a former United States Peace Corps volunteer. Find him on Instagram @dr.daignault