"Unless you are an elite athlete, it doesn't matter," said Miriam Nelson, director of the John Hancock Center for Physical Activity and Nutrition at Tufts University, in an e-mail. "The most important thing is to find a time of day that you like best and when you can exercise most consistently."
And this advice is especially true for aerobic exercise like brisk walking or cycling. For aerobic workouts, "it doesn't much matter when you do it. Basically, it's whenever you can," said Michael R. Deschenes, a physiologist and neurobiologist at the College of William and Mary in Virginia.
For building muscle strength through weight training, though, "you're better off in the late afternoon or early evening because core body temperature is higher and muscles perform better when core temperature is higher," Deschenes said.
The most important message, said Deschenes, is consistency: "Exercise regularly whenever you can at moderate intensity."
So far, most of the research on exercise and "chronobiology," the study of biologic rhythms over a 24-hour cycle, has been done in men, and relatively young men at that. There's some evidence, said Deschenes, that the timing of exercise may matter even less with age "because circadian rhythms get less pronounced as you get older."
Even so, the basic idea is still to exercise when it suits your own schedule.
"If you are an evening person and someone tells you to exercise in the morning, you will never enjoy it (and not stay with it). So the best message is to exercise when you feel most comfortable," said William J. Evans, a physiologist at the University of Arkansas for Medical Sciences.
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