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Back in 1987, University of Toronto researcher Dr. Nicholas Mrosovsky put a group of hamsters through an eight-hour time change, and then made half of them run on an exercise wheel in the new time zone while the others slept. The result: The exercised hamsters adjusted to the new time zone in 1.5 days on average, while the sleepers took 8.5 days.

If you could put that result into a pill, it would be a best-seller.

But humans aren't hamsters, and for more than two decades, researchers have struggled to turn Mrosovsky's initial observation into a usable prescription for resetting your body's internal clock. Now a series of new studies offers fresh clues about how regular, strategically scheduled exercise can help you adjust to new time zones – and perhaps, more importantly, strengthen circadian rhythms that weaken as we age.

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The recent switch to daylight time was a reminder that adjusting your internal clock can be a difficult and sometimes unpleasant task. Rotating shift workers and frequent travellers know this all too well – but all of us experience progressively weakening internal rhythms as we get older, disrupting sleep schedules and making it harder to bounce back from time shifts. The stakes are high: Persistent disruptions in circadian rhythms are linked to diabetes, cancer progression, memory and cognition problems, and a host of other health concerns.

One of the key questions that remain unanswered is what time of day is best to exercise. Researchers from UCLA tackled this issue in a study published in the Journal of Physiology, in which they assigned mice to exercise either early or late during the night, or whenever they wanted. (Since mice are nocturnal, their night hours are analogous to human daytime hours.)

As you might expect, early exercise shifted circadian cycles like heart rate and body temperature to peak earlier in the day, while late exercise shifted those same peaks to later in the day. This suggests that morning workouts might help those who are flying east, while afternoon workouts would help those flying west.

More surprisingly, the results showed that the mice working out later in the night received a bigger overall boost in the functioning of their internal clocks compared to the early workout group – a result that the researchers were unable to explain. Could it be that, in humans, afternoon workouts are most effective for synchronizing your body in a new time zone? Only further studies will tell, the researchers say.

Despite the unanswered questions, a forthcoming study in the journal Age offers a more practical message. Researchers from Amherst College, Mass., compared the function of the "suprachiasmatic nucleus," or SCN – the body's central timekeeper, located in the brain – in young and old mice, and saw significant differences.

"We think that the output signal from the SCN is weakened by aging," explains Dr. Mary Harrington, the senior author of the study.

In other words, the clock is still ticking properly, but its message isn't reaching the rest of the body as effectively as it used to. But when the mice were given regular access to an exercise wheel, this age-related decline in SCN function was slowed, and even at the geriatric-for-mice age of 18 months, they were able to adjust more quickly following an eight-hour time change.

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The use of targeted workouts at specific times to tackle jet lag remains an open question, but Harrington's research points to a much more broadly applicable message: Regular, consistent exercise will keep your circadian clock ticking strongly, helping you adjust to time changes and protecting you from a subtle but damaging side-effect of aging.

In the battle against wayward internal clocks, sleeping pills, sedatives and hormones like melatonin are often deployed with limited and variable success. As Mrosovsky and his co-author Peggy Salmon wrote in their seminal paper 26 years ago, "For the fit and pharmacologically conservative, jogging might be preferable to drugs."

Editor's note: An earlier version of this article stated the wrong directions when advising travellers of when to exercise. This has been corrected.

Tips to keep your clock ticking

Exercise regularly at a time that's comfortable for you. Avoid exercise after 11 p.m.

Restrict your eating to a 12-hour period during the day – for example, 7 a.m. to 7 p.m.

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Avoid light exposure, such as bright electronic screens, just before bed and at night.

Alex Hutchinson blogs about exercise research at His latest book is Which Comes First, Cardio or Weights?

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About the Author
Jockology columnist

Alex Hutchinson writes about the science of fitness and exercise. A former national-team distance runner and postdoctoral physicist, he is the author of Which Comes First, Cardio or Weights? Fitness Myths, Training Truths, and Other Surprising Discoveries from the Science of Exercise. He is also a senior editor at Canadian Running magazine and a contributing editor at Popular Mechanics. More


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