And people who hit the gym or pound the pavement long after sundown may actually be setting themselves up for a tough night.
In a study printed in the December Journal of Physiology, researchers used mice to see how exercise affects the body's internal clock.
Scientists thought they would see the best results from morning exercise.
But instead, they found mice that ran later produced more proteins that benefit the internal clock.
The New York Times reports that recent studies have linked a disruption in circadian rhythm to increased risk of obesity, depression, memory loss, diabetes and even types of cancer.
Researchers say it isn't clear if an afternoon jog is really better than a morning one.
But they say a late-night workout seems to lead to a significant disruption in the circadian rhythm.