Is There an Optimal Time of Day to Exercise?

Oct 29, 2016, 02:00 PM

Exercise is wonderful in any amount, and may even be life-saving, but there’s an optimal time for it, suggests new research published in Cell Metabolism.

Northwestern University researchers studied the muscle cells of mice, specifically looking at their internal clock, or circadian rhythm. Their initial experiments in disrupting the internal clock genetically hampered the cells’ ability to burn fuels like sugar and fat in response to low oxygen concentrations—important when exercising vigorously. Secondly, they found that the cells best burned fuel during the earliest hours of the mice's normal waking period. While mice are nocturnal, the researchers believe their findings could apply in reverse to people, as well.

“Oxygen and the internal clock are doing a dance together inside muscle cells to produce energy, and the time of day determines how well that dance is synchronized,” said senior author Dr. Joseph Bass, a professor of endocrinology in the Department of Medicine at Northwestern, in a statement. “The capacity for a cell to perform its most important functions, to contract, will vary according to the time of day.”

When we lightly exercise, our cells churn through oxygen to give us the necessary energy, but as a workout intensifies and there’s less oxygen around, we turn to our sugar and eventually fat reserves. The oxygenless source of fuel generates a buildup of lactic acid, which acts as a sort of safety brake—it lets us exert ourselves to a certain point before it forces us to slow down and catch our breath (and oxygen) again. But this process went haywire in mice when the researchers mucked with the cells’ internal clock.

“When we manipulated the clock genetically, we noticed there were profound abnormalities in the muscle,” Bass explained. “That set us on a course to understand how the inner muscle clock is important in regulating how well the muscle cell can mobilize energy.”

Bass and his colleagues are cautious about speculating too far ahead without more research.

“We’re not saying we can tell athletes when they should work out,” said Bass. “But in the future, perhaps, you may be able to take advantage of these insights to optimize muscle function.”

Understanding how the proteins that govern our body clock interact with muscle cells may also someday allow us to manipulate the cells’ oxygen response, Bass added. That in turn could lead to insights in metabolic disorders like diabetes.