Exercise can help us sleep better and reduce our risk for chronic disease—but does it matter when you do it?
New research on mice from the University of California, Los Angeles’s Brain Research Institute suggests that exercising during the daytime can improve your sleep and reduce your risk for health problems that are associated with a disrupted internal clock, like fatigue during the day, difficulty sleeping, weight gain, and dysfunction of the cardiovascular and immune systems. What they did not find, as some media outlets reported, is that the afternoon is the best time to exercise in order to reap these benefits.
Researchers observed several groups of mice running in their wheels. Some were otherwise healthy and some were bred to have a malfunctioning internal clock, or circadian timing system. Some mice could run whenever they wanted, while others only had access to the wheel at the mouse equivalents of morning and afternoon (they’re nocturnal). Exercise improved the functioning of the internal clock in all of the mice, but in the mice with “broken” clocks, the effect was more pronounced in the afternoon.
Lead author Christopher Colwell, Ph.D., who has studied circadian rhythms for 30 years, says that our brain’s internal clock governs most aspects of our behavior and physiology by telling our cells what time it is and what they should be doing, like maintaining organ function (daytime) or going into repair mode (nighttime). Aging, nervous system diseases, and exposure to artificial light at night can all disrupt our circadian rhythm and, he says, “disruption of the clock has profound influence throughout the whole body.”
Colwell says this study raises the possibility that there may be a difference in how exercise in the morning versus the afternoon affects the clock in humans, but he’s not aware of any literature on that premise.
“I’ve been getting some emails from people who exercise in the morning and they feel great and they’re saying ‘Well, should I change that?’ Absolutely not,” he says.
“Right now, we feel comfortable saying that exercise during a human’s daytime would be beneficial, while the same exercise during the normal sleep time would be disruptive to these rhythms.”
This study, which appears in the Journal of Physiology, did not examine the effects of late-night exercise, but unpublished results from Colwell’s lab show that working out at the equivalent of 11pm disrupts the clock.
Colwell says that sweating in the morning and late afternoon and maybe even the early evening are perfectly fine, but he says, “I would caution, as I have observed anecdotally, that if your only option is exercising at midnight, you might want to skip it that day.” It can affect your sleep and throw off the clock: We get sleepy when our internal body temperature starts to decline and vigorous exercise causes a spike in core temperature, which delays the process.
“But, of course, we still want people to exercise.”
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