Your body has a circadian clock that reacts to the time of day, and researchers from Northwestern University theorized that muscle tissue might work this way. This is because muscle cells operate more efficiently during the hours you are normally awake. The study was published in the journal Cell Metabolism.

Researchers had lab mice exercise at different times of day. Their muscles turned on genes that help with the adaptation to exercise at night, which is not surprising since mice are nocturnal. Although they can’t say for sure what time of day humans might be able to get the most out of exercise, it’s likely to be at some point between sunrise and sunset. Further research might bring a more conclusive answer.

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