It found that, for women, morning workouts zapped abdominal fat and improved blood pressure better than late-day training. For men, evening exercise led to greater fat burning and better blood pressure control. Evening exercise also amplified the benefits of strength training, but more so for women.
Studies of exercise timing are part of the burgeoning science of chronobiology, which focuses on how our internal clocks affect almost every aspect of our physiology.
Human bodies, like those of other mammals, plants, reptiles and insects, operate on an innate 24-hour circadian rhythm, with a master clock system in our brains sending and receiving biochemical signals that coordinate with molecular clocks inside our cells to direct a boggling symphony of biological processes.
This rhythm, in turn, responds to signals from the outside world, especially daylight and darkness, but also when we eat, sleep and exercise.
Recent studies in mice allowed large groups of rodents to run on exercise wheels at varying times of day. The studies showed that the animals’ heart rates, fat burning, gene expression and body weights change substantially, depending on when they exercise — even if the exercise itself is the same.
Human studies of exercise timing have been more contradictory, though. Some show people burn extra fat and lose more weight by exercising early, especially before breakfast, while others suggest we gain greater health benefits from afternoon or evening workouts.
But most of these studies were small and involved only men with metabolic conditions, such as Type 2 diabetes, high blood pressure or obesity. So we have known little about optimal exercise timing for healthy men — and even less about the best timing for women. Which is why the new study is so meaningful.
A real-world study of exercise timing
Published in May in Frontiers in Physiology, the research was designed to reflect real-world demographics, said Paul Arciero, the director of the Human Nutrition, Performance & Metabolism Laboratory at Skidmore College, in Saratoga Springs, N.Y., and the study’s lead author.
All of the volunteers self-identified as male or female, and more than half of the 56 participants were women. They also were all healthy and physically active but not athletes.
The researchers tested the volunteers’ health, strength and fitness, then randomly divided them into two groups, with equal numbers of men and women. One group was asked to exercise four times a week in the morning, between 6 a.m. and 8 a.m. The other group was instructed to exercise in the evening, between 6:30 and 8:30 p.m.
Each group took part in an identical workout. Once a week, they lifted weights. The next day, they did about 35 minutes of interval training (running, swimming or cycling as hard as possible for about a minute, resting and repeating). Another day, they did yoga or Pilates. They ended the week with about an hour of running, cycling or other aerobic exercise.
The groups maintained this routine for 12 weeks, then returned to the lab for retesting.
Everyone in the study was leaner, faster, fitter, stronger, healthier and more flexible, whether they worked out early or late.
Do you want to shed belly fat? Or build strength?
But there were relevant differences between the groups based on what time of day they exercised. Here’s what the researchers found:
- For women, fat burned away better in the morning. The female early exercisers dropped, on average, about 3 percent more total body fat than the evening exercisers, with much of the loss coming from their waistlines. The morning exercisers shed about 7 percent more abdominal fat than women working out in the evening. (None of the volunteers’ total body weight fell, since they gained muscle as they lost fat.)
- Morning exercise also lowered blood pressure in female exercisers significantly better than the same workouts in the evening.
- Evening exercise by women, meanwhile, amplified strength gains. Overall, the evening exercisers improved their upper body strength by 7 percent more than the morning group, and they also powered through more sit-ups and push-ups.
- For men, evening exercise was the clear winner in terms of health. The evening exercisers significantly lowered their cholesterol levels, while the morning exercisers, surprisingly, slightly raised theirs. Evening exercise also fired up men’s fat-burning. By the end of the study, the bodies of the male evening exercisers were burning about 28 percent more fat during workouts than at the start, a shift that can drive body-fat loss. The morning group’s fat-burning grew only slightly.
- Any time was the right time, though, for men to up their strength and fitness. Among the men, morning and evening exercisers raised their bench press, leg press, sit-up, push-up and other strength bests by about the same extent, whether they exercised early or late.
What these results mean in practical terms is that women with specific health or fitness goals may want to finesse the timing of their workouts, Arciero said. If you are a woman hoping to lose inches around the middle, consider morning workouts. If your goal is strength, evening workouts might be more effective.
For men, exercising early or late seems comparable for strength and fitness, but evening exercise could have special advantages for health, Arciero said.
Still, “it is early days yet with regard to providing individualized prescriptions for the optimal time of day to exercise,” said John Hawley, the head of the Exercise and Nutrition Research Program at the Australian Catholic University in Melbourne, Australia, who has extensively studied exercise metabolism and timing but was not involved with this study.
He pointed out that the new study did not control for women’s menstrual cycles or track people’s chronotypes — whether they naturally were morning or evening people — both of which could influence exercise responses. It also did not include midday exercise or look into why men and women reacted so differently to exercise timing. Arciero suspects hormones and other cellular and genetic effects, and plans follow-up studies to learn more, he said.
For now, the study’s key takeaway is that timing may fine-tune what we gain from exercise. But we benefit, regardless, so, “any time of day that you choose to exercise is the right time,” Hawley said.
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