Is it better to work out in the morning or evening? It’s an age-old debate that often concludes with something along the lines of, “it doesn’t matter when you work out, just that you do so on a consistent basis.”

But now, scientists have found that morning and evening workouts have different benefits, and that your goals could determine when you should hit the gym. According to a recent study from the University of Copenhagen in Denmark, working out in the morning results in improved metabolism of sugar and fat, whereas evening training sessions result in more overall burnt calories.  

“There appear to be rather significant differences between the effect of exercise performed in the morning and evening, and these differences are probably controlled by the body’s circadian clock,” said Jonas Thue Treebak, one of the researchers from Novo Nordisk Foundation Center for Basic Metabolic Research, according to a press release. The research will be published in the next issue of Cell Metabolism 

There is one caveat to the study—it was conducted on mice, not humans. The scientists, though, said they’re “eager” to do a similar study with humans, according to Treebak. Researchers had the rodents exercise at the start and end of the day, then examined the muscle cells after each session.

Mice that exercised in the morning were able to metabolize sugar and fat better than those that had evening workouts. This means that an early-morning gym stop would be beneficial to severely overweight people or those with type 2 diabetes. On the other hand, evening exercise was better for overall energy expenditure. The mice that worked out at night were able to burn more calories, even for several hours after they finished their exercise.

The scientists stressed that they could not say which time of day was better to work out. “At this point, we can only conclude that the effects of the two appear to differ, and we certainly have to do more work to determine the potential mechanisms for the beneficial effects of exercise training performed at these two time-points,” Treebak said.