Morning guys get out at the butt crack of dawn. Day guys try to squeeze something in at lunch. (Good luck with that.) Late guys hit it once the work whistle blows. But it’s all good as long as the workouts are consistent, right?

Maybe not. According to a recent study out of Northwestern University, muscle tissue runs on the same circadian clock—the internal 24-hour timer that regulates cell functioning—that the rest of the body does. If true, that means there’s an optimal time for muscle cells to use oxygen to create energy and allow for better performance.

The study found that when the muscles in mice were genetically altered to ignore the little rodents’ circadian clock, they lost their ability to use sugar for energy. They also stopped creating lactic acid, the by-product of intense exercise that allows glucose breakdown and causes that burning sensation in your muscles.

Knowing when muscles will work at their optimum output could help scientists develop techniques for maximize power, they speculate.