The benefits of exercise may differ depending on the time of day when you work out, a new study in mice suggests.

Too little sleep can have severe health consequences, but researchers are still making discoveries confirming that the body’s circadian clock affects our health.

The new research with mice shows that the effect of exercise performed in the beginning of the mouse’ dark/active phase—corresponding to our morning, differs from the effect of exercise performed in the beginning of the light/resting phase—corresponding to our evening.

“There appears 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,” says Jonas Thue Treebak, associate professor at the Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen.

“Morning exercise initiates gene programs in the muscle cells, making them more effective and better capable of metabolizing sugar and fat. Evening exercise, on the other hand, increases whole body energy expenditure for an extended period of time.”

For the study in Cell Metabolism, researchers measured a number of effects in the muscle cells, including the transcriptional response and effects on the metabolites. The results show that responses are far stronger in both areas following exercise in the morning. A central mechanism involving the protein HIF1-alfa, which directly regulates the body’s circadian clock, likely controls this, the researchers say.

Morning exercise appears to increase the ability of muscle cells to metabolize sugar and fat. This type of effect interests researchers in relation to people with severe overweight and type 2 diabetes.

On the other hand, exercise in the evening increases energy expenditure in the hours after exercise. Therefore, the researchers cannot necessarily conclude that exercise in the morning is better than exercise in the evening, Thue Treebak says.

“On this basis we cannot say for certain which is best, exercise in the morning or exercise in the evening. 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.

“We are eager to extend these studies to humans to identify if timed exercise can be used as a

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