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The time of day you exercise might affect what kind of results you see from your efforts.

Two new studies appear to confirm that your circadian clock is a factor in how the body responds to physical exertion.

Both papers were published April 18 in the journal Cell Metabolism. And because they focused on different components of exercise, Science Daily noted, they complement each other.

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“It’s quite well known that almost every aspect of our physiology and metabolism is dictated by the circadian clock,” said Gad Asher of the Department of Biomolecular Sciences at the Weizmann Institute of Science, who is senior author of one of the studies. “This is true not only in humans but in every organism that is sensitive to light. We decided to ask whether there is a connection between the time of day and exercise performance.”

Paolo Sassone-Corsi added: “Circadian rhythms dominate everything we do. Previous studies from our lab have suggested that at least 50% of our metabolism is circadian, and 50% of the metabolites in our body oscillate based on the circadian cycle. It makes sense that exercise would be one of the things that’s impacted.”

Sassone-Corsi, senior author of the other paper, is with the Center for Epigenetics and Metabolism at the University of California, Irvine.

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To explore the association between time of day and exercise performance, both teams observed mice. Because mice are nocturnal, however, they had to convert mouse timing to human timing, by distinguishing between the active phase and resting phase of the mice rather than using numbers on the clock.

Asher’s team put their mice on treadmills at various times throughout their active phase. They found that overall exercise performance was about 50% better on average in the “mouse evening” (toward the end of their active time) compared to the morning hours. These daily differences were diminished in mice that had mutant clocks — supporting a potential role of the clock in the observed variance in exercise performance.

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Sassone-Corsi’s team also put mice on treadmills, but they looked at processes like glycolysis (which contributes to sugar metabolism and energy production) and lipid oxidation (fat burning).

They found that a protein called hypoxia-inducible factor 1-alpha (HIF-1) is activated by exercise in different ways depending on the time of day. HIF-1? is a transcription factor that is known to stimulate certain genes based on oxygen levels in tissue.

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“It makes sense that HIF-1 would be important here, but until now we didn’t know that its levels fluctuate based on the time of day,” Sassone-Corsi said. “This is a new finding.”

Exercise seemed to have the most beneficial effect on metabolism at the beginning of the active phase (equivalent to late morning in humans) compared with the resting phase (evening).

The researchers noted that translating the findings to humans is not so straightforward. One reason is that humans have more variation in their chronotypes than mice living in a lab. 

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“You may be a morning person, or you may be a night person, and those things have to be taken into account,” Sassone-Corsi said.