What is it about?

Physical activity affects the body differently depending on the time of the day. That’s because of the genetic clocks inside almost all cells, which regulate biological processes over a 24-hour period, otherwise called a cell’s circadian rhythm. We studied the fat cells, or adipose tissue, of mice after a session of high-intensity exercise performed at two different times of day – at the early active phase and the early rest phase, which correspond to a late morning and late evening session, respectively, in humans. We looked for different markers that indicated fat burning analyzed which genes were active in adipose tissue after exercise.

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Why is it important?

We found found that physical activity during the early active phase activated genes involved in the breakdown of adipose tissue, heat production and mitochondria in the adipose tissue. These all indicate a higher metabolic rate, and were only observed in mice that exercised in the early active phase and were independent of food intake. Our results suggest that late morning exercise could be more effective than late evening exercise in terms of boosting the metabolism and the burning of fat, and if this is the case, they could prove of value to people who are overweight.


The right timing seems to be important to the body’s energy balance and to improving the health benefits of exercise, but more studies are needed to draw any reliable conclusions about the relevance of our findings to humans.

Juleen Zierath
Karolinska Institutet

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This page is a summary of: Time of day determines postexercise metabolism in mouse adipose tissue, Proceedings of the National Academy of Sciences, February 2023, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2218510120.
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