Does it make a difference? Several recent studies suggest that it does. But it’s complicated.
One recent paper indicates that morning exercise may activate certain genes in the muscle cells, boosting their ability to metabolize sugar and fat. While scientists say this finding requires further study, they think it ultimately might help those who are overweight or suffering from Type 2 diabetes.
An evening workout, on the other hand, uses less oxygen, making workouts more efficient and improving athletic performance, potentially a boon for serious competitors.
“Human exercise performance is better in the evening compared to the morning, as [athletes] consume less oxygen, that is, they use less energy, for the same intensity of exercise in the evening versus the morning,” said Gad Asher, a researcher in the Weizmann Institute of Science’s department of biomolecular sciences, and author of one of the studies.
“It means, for example, if a person needs to go for a run, he will reach exhaustion earlier in the morning compared to the evening,” Asher said. “In other words, he will be able to run for a longer duration in the evening compared to the morning under the same running conditions.”
So which is the better time to exercise — morning or evening?
It depends on your goals.
Elite and otherwise serious athletes — marathon runners, basketball and soccer players seeking a competitive edge, for example — might choose evenings to train or compete. Similarly, those who schedule important sports events might consider holding them at night to ensure optimal performances. (Not to mention that this also would probably be more compatible for TV coverage.)
“If you wish to break the world record, or your personal time, I assume [evenings would be better],” Asher says.
Those who worry more about their weight and controlling their blood sugar — and less about shaving a minute or two from their marathon time — might go for mornings, when post-workout cell responses that influence metabolism are much stronger.
Jonas Thue Treebak, associate professor at the Novo Nordisk Foundation’s Center for Basic Metabolic Research at the University of Copenhagen and co-author of a third study, agrees.
“At this point, we can only conclude that the effects of the two appear to differ, and we certainly have to do more work,” he says.
Moreover, there are other things beyond performance and weight loss to take into account.
“Exercising late at night may interfere with sleep as it tends to energize you and enhance alertness, although some people like to exercise at the end of the day to help relieve the stresses of the day and prepare for evening activities, which is fine,” says Edward R. Laskowski, co-director of the Mayo Clinic Sports Medicine and professor in the department of physical medicine and rehabilitation. “Morning exercise has the advantage that no matter what else happens during the day, you have incorporated your physical activity. It also increases alertness and helps cognitive functioning.”