New research identifies best exercise times for adjusting body's internal clock

Everyone needs sleep, but that doesn’t mean it comes easy for everyone — especially when your natural sleep cycle is disrupted, throwing off your internal clock.

New research, though, offers hope for people looking to adjust more easily to unique bedtimes associated with jet lag, shift work or military deployments.

ASU College of Nursing and Health Innovation Professor Shawn Youngstedt and his co-authors Jeffrey Elliott and Daniel Kripke wanted to expand on previous research that had shown exercising can cause changes to the body clock or circadian rhythm.

“We know that it can affect the internal clock, but there was never a clear understanding of what time of day exercise causes delays and when exercise advances the body clock. Without knowing this information, it is more difficult to help people who have body-clock disturbances,” he said.

So their study, outlined in an article just published in the Journal of Physiology, sought to narrow down the time of day you should work out for the desired adjustment.

Their results found:

• Exercise at 7 a.m. or between 1 and 4 p.m. advanced the body clock, which would help people start activities earlier the next day.

• Exercise between 7 and 10 p.m. delayed the body clock, which would help people shift their peak performance later the next day.

• Exercise between 1 and 4 a.m. or at 10 a.m. had little effect on the body clock.

We asked Youngstedt to walk us through the study, the findings and to put the challenges of body-clock disruption in perspective.

Question: What makes this study unique?

Answer: This was the first study to examine exercise at eight different times of the day or night in a large enough number of subjects (101 subjects) to show clearly when exercise advances the body clock and when it delays the body clock. This was also the first study to compare women vs. men and older adults vs. young adults. No differences were found in how exercise shifts the body clock by age or sex.

Shawn Youngstedt

Q: What are some of the common disruptors to one's internal body clock?

A: Shift work and jet lag are common disruptors. Having a light on, even a cellphone light, at night can delay the body clock, making it harder to get up in the morning. Not getting enough outdoor light or physical activity are also disruptors.

Q: What can happen if you experience those disruptors?

A: In the short term, these disruptors often lead to sleep disturbance, impaired mood and alertness, and increased risk of accidents. Jet lag also commonly leads to digestion problems. Shift work is associated with a high risk of cancer. Indeed, shift work is now considered a carcinogenic behavior. It is also associated with cardiovascular disease, depression, diabetes and obesity.

Q: How significant a problem is this?

A: About 20 percent of the world's population are shift workers, and millions of air travelers suffer from jet lag annually. Social jet lag, which is associated with sleeping later and longer on weekends than on school or work days, also seems to be becoming more prevalent. Delayed sleep phase is particularly common in adolescents and young adults and commonly leads to loss of sleep during the week.

Q: What’s the potential impact of your study’s findings?

A: We think there will be more exploration of exercise for shifting the body clock in real-world situations.

Q: Are there plans to further the research?

A: We hope to look at dose-response effects. Our study used one hour of moderately intense exercise. It will be helpful to know if similar effects are shown with lighter-intensity exercise or shorter exercise. Maybe slowly strolling around the Louvre for a few hours would produce the same or greater effects, which would be helpful to know for travelers. Another thing we would like to explore is combining exercise with bright light and melatonin. Now that we know the best time for causing shifts for all of these stimuli, we might be able to travel across 12 time zones and be adjusted in just a few days.