LEILA FADEL, HOST:
When you work out impacts how you sleep, as NPR's Will Stone reports.
WILL STONE, BYLINE: Don't worry, science is not coming after your favorite workout time - at least, not yet - because so far, there's no real consensus about the best time to work out. Katja Lamia, a circadian biologist at Scripps Research, says there's one thing that's absolutely clear.
KATJA LAMIA: Truly, I think the best time to exercise is whenever you're actually going to do it.
STONE: When to work out is actually not at all a straightforward question. Who's exercising? What are you doing?
LAMIA: Definitely depends on what your goals are.
STONE: There is consistent evidence that athletic performance, including strength, peaks in the afternoon. This is when world records tend to be broken. But Raphael Knaier says the existing research is still limited and mostly focuses on young men, so it's hard to generalize. Knaier is an exercise physiologist at the University of Basel.
RAPHAEL KNAIER: The afternoon is the best time on average over a certain population - does not mean that it's the best time for every individual.
STONE: In fact, in one study he did measuring strength and time of day, he asked athletes when they thought their peak performance was.
KNAIER: And almost none of them guessed correctly.
STONE: Exercise timing isn't just relevant for elite competitors. Physical activity, while not as powerful as light, can also influence our circadian rhythms and help keep our body in tune. This extends beyond the central clock and the brain to the peripheral clocks that populate our tissues, from the liver to the heart to our muscles. Karyn Esser has focused on the interplay between muscle clocks and exercise. She's a physiologist at the University of Florida's College of Medicine.
KARYN ESSER: If we're healthy, our clocks are kind of all aligned. You know, they're all in the same time zone. And the problem arises when our clocks get out of phase. You know, the brain clock thinks it's here, and the liver and the pancreas think it's something else.
STONE: This circadian misalignment is linked to chronic diseases, and most of us know the crummy feeling of jetlag. Esser says the muscle clocks steer key functions related to energy and metabolism that fluctuate.
ESSER: It's really part of homeostasis or taking care of business in the cell.
STONE: Meaning it's telling your muscles when they should be primed to go and when they should rest and repair.
ESSER: There is a contribution of the muscle clocks to the muscle performance and to the athletic performance.
STONE: To what extent is hard to say. Research shows the muscle clocks are quite responsive to exercise, and they can adapt.
ESSER: What time those muscles are active actually talks to the muscle clock and helps adjust its phase independent of whatever it's getting from the central clock.
STONE: Of course, muscles aren't the only part of the body involved in exercise, and there are other variables, like whether you tend to be an early riser or late riser. Generally speaking, Esser says it's probably better if you aim to exercise around the same time every day. There's also growing interest in how exercise timing can unlock added health benefits, especially for certain medical conditions. This is what Juleen Zierath is focused on.
JULEEN ZIERATH: What we're trying to understand is how can you fine-tune the exercise prescription?
STONE: Zierath is a professor of physiology at the Karolinska Institute in Sweden. She studied the effect of high-intensity training in men with Type 2 diabetes and found an afternoon session improved their blood sugar.
ZIERATH: And surprisingly, when the men performed the exercise in the morning, these same men had a worsening of their blood glucose control.
STONE: She says it's still early days for this type of research, though. And ultimately, it seems like you can't go wrong with any exercise anytime.
Will Stone, NPR News.
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