The Best Time of Day to Exercise for Metabolic Health

The exercise routines were identical, intermingling brief, intense intervals on stationary bicycles one day with easier, longer workouts the next. The exercisers worked out for five consecutive days, while continuing the high-fat diet. Afterward, the researchers repeated the original tests.

The results were somewhat disturbing. After the first five days of fatty eating, the men’s cholesterol had climbed, especially their LDL, the unhealthiest type. Their blood also contained altered levels of certain molecules related to metabolic and cardiovascular problems, with the changes suggesting greater risks for heart disease.

Early-morning exercise, meanwhile, did little to mitigate those effects. The a.m. exercisers showed the same heightened cholesterol and worrisome molecular patterns in their blood as the control group.

Evening exercise, on the other hand, lessened the worst impacts of the poor diet. The late-day exercisers showed lower cholesterol levels after the five workouts, as well as improved patterns of molecules related to cardiovascular health in their bloodstreams. They also, somewhat surprisingly, developed better blood-sugar control during the nights after their workouts, while they slept, than either of the other groups.

The upshot of these findings is that “the evening exercise reversed or lowered some of the changes” that accompanied the high-fat diet, says Trine Moholdt, an exercise scientist at the Norwegian University of Science and Technology, who led the study in Australia as a visiting researcher. “Morning exercise did not.”

This study does not tell us how or why the later workouts were more effective in improving metabolic health, but Dr. Moholdt suspects they have greater impacts on molecular clocks and gene expression than morning exertions. She and her colleagues hope to investigate those issues in future studies, and also look at the effects of exercise timing among women and older people, as well as the interplay of exercise timing and sleep.

For now, though, she cautions that this study does not in any way suggest that morning workouts aren’t good for us. The men who exercised became more aerobically fit, she says, whatever the timing of their exercise. “I know people know this,” she says, “but any exercise is better than not exercising.” Working out later in the day, however, may have unique benefits for improving fat metabolism and blood-sugar control, particularly if you are eating a diet high in fat.