The effect of exercise may differ depending on the time of day

Time of day could affect the body's response to exercise.


Finding the perfect time to exercise is as much about personal preference as it is physiology. Exercise is supposed to feel good—but if muscles are tight in the morning or working out too late disrupts sleep, it can feel counterproductive.

We all know we should exercise regularly, but it can be challenging to fit exercise into our busy schedules. Most people can only exercise before or after work, so it’s worth examining whether the time of day we exercise affects outcomes such as weight loss and sleep.

In a new study, scientists have learned that the effect of exercise may differ relying on the time of day. They found that exercise in the morning results in an increased metabolic response in skeletal muscle, while exercise later in the day increases energy expenditure for an extended period.

Scientists at the University of Copenhagen – in collaboration with researchers from University of California, Irvine conducted the study on mice and found that the effect of exercise performed at the beginning of the mouse’ dark/active phase, corresponding to our morning, differs from the impact of exercise performed at the beginning of the light/resting phase, corresponding to our evening.

Associate Professor Jonas Thue Treebak from the Novo Nordisk Foundation Center for Basic Metabolic Research said, “There appear to be rather significant differences between the effect of exercise performed in the morning and evening, and the body’s circadian clock probably controls these differences. 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.”

Scientists quantified numerous effects in muscle cells, including transcriptional response and impact on the metabolites. They found that responses are far stronger in both areas following exercise in the morning and that this is likely to be controlled by a central mechanism involving the protein HIF1-alfa, which directly regulates the body’s circadian clock.

At the molecular level, HIF1α, a central regulator of glycolysis during hypoxia, is selectively activated in a time-dependent manner upon exercise, resulting in carbohydrate exhaustion, usage of alternative energy sources, and adaptation of systemic energy expenditure.

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

On the other hand, the results also show that 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 an exercise in the evening.

Jonas Thue Treebak said, “On this basis, we cannot say for certain which is best, an 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 treatment strategy for people with metabolic diseases.”

The study is published in the journal Cell Metabolism.

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