The circadian clock is the 24 hour cycle that regulates many physiological processes including sleeping and eating. Many factors affect this internal clock including light and time cues. According to a study published online this month in the Journal of Physiology, exercise can shift the circadian clock, with the direction and amount of this effect depending on the time of day or night in which people exercise.

“Exercise has been known to cause changes to our body clock,” said study lead author Dr. Shawn Youngstedt, a researcher in the College of Nursing and Health Innovation at Arizona State University.

“We were able to clearly show in this study when exercise delays the body clock and when it advances it.”

Dr. Youngstedt and his colleague from the University of California, San Diego, found that exercising at 7 a.m. or between 1 and 4 p.m. advanced the body clock to an earlier time, and exercising between 7 and 10 p.m. delayed the body clock to a later time.

Exercising between 1 and 4 a.m. and at 10 a.m., however, had little effect on the body clock, and the phase-shifting effects of exercise did not differ based on age nor gender.

The scientists examined body clocks following exercise in 101 participants for up to 5.5 days.

The baseline timing of each participant’s body clock was determined from urine samples collected every 90 min to measure the time of the evening rise in melatonin and the peak of melatonin several hours later.

Participants then walked or ran on a treadmill at a moderate intensity for one hour per day for three consecutive days.

They exercised at one of eight different times of day or night, but each individual exercised at the same time on all three days or nights.

The timing of the body clock was re-assessed following the third exercise session.

“This is the first study to compare exercise’s effects on the body clock, and could open up the possibility of using exercise to help counter the negative effects of jet lag and shift work,” Dr. Youngstedt said.

“Further research will look at combining exercise with bright lights and melatonin to see what impact this has on body clocks, as well as examining the effect of changes in exercise duration and intensity on the body clock.”

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Shawn D. Youngstedt et al. Human Circadian Phase-Response Curves for Exercise. Journal of Physiology, published online February 19, 2019; doi: 10.1113/JP276943