He found that, for women, morning workouts shed abdominal fat and improved blood pressure better than late-day workouts. For men, evening exercise resulted in greater fat burning and better blood pressure control. Evening exercise also magnified the benefits of strength training, but more so for women.
Exercise timing studies are part of the emerging 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 that sends and receives biochemical signals that coordinate with clocks. molecules inside our cells to direct a bewildering symphony of biological processes.
This rhythm, in turn, responds to cues from the outside world, especially daylight and darkness, but also when we eat, sleep, and exercise.
Recent mouse studies have allowed large groups of rodents to run on exercise wheels at different times of the day. Studies have shown that the heart rate, fat burning, gene expression and body weight of animals change dramatically depending on when they exercise, even if the exercise itself is the same.
Human studies on the timing of exercise, however, have been more contradictory. Some show that people burn more fat and lose more weight by exercising early, especially before breakfast, while others suggest that afternoon or evening workouts provide greater health benefits.
But most of these studies were small and only involved men with metabolic disorders, such as type 2 diabetes, high blood pressure or obesity. So we know little about the optimal timing of exercise for healthy men – and even less about the best time for women. This is why the new study is so significant.
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, director of the Human Nutrition, Performance & Metabolism Laboratory at Skidmore College, Saratoga Springs, NY, and lead author of the ‘study.
All volunteers identified as either male or female, and more than half of the 56 participants were female. They were also all healthy and physically active, but not athletes.
The researchers tested the health, strength and physical condition of the volunteers, then randomly divided them into two groups, with an equal number 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 asked to exercise in the evening, between 6:30 p.m. and 8:30 p.m.
Each group participated in identical training. Once a week they lifted weights. The next day, they did about 35 minutes of interval training (run, swim, or bike as hard as they could for about a minute, rest, and repeat). Another day they did yoga or Pilates. They ended the week with about an hour of running, biking or other aerobic exercise.
The groups maintained this routine for 12 weeks and then returned to the lab for retesting.
Everyone in the study was leaner, faster, fitter, stronger, healthier and more flexible, whether they train sooner or later.
Do you want to get rid of belly fat? Or build strength?
But there were relevant differences between the groups based on the time of day they exercised. Here is what the researchers found:
- For women, fat burns best in the morning. Women who exercised early lost an average of about 3% more total body fat than those who exercised in the evening, with much of the loss coming from their waistline. People who exercise in the morning lose about 7% more belly fat than women who exercise in the evening. (None of the volunteers’ total body weight decreased, as they gained muscle while losing fat.)
- Morning exercise also lowered blood pressure in women performing much better than the same workouts in the evening.
- Evening exercise by women, meanwhile, amplified strength gains. Overall, the evening practitioners improved their upper body strength 7% more than the morning group, and they also performed more sit-ups and push-ups.
- For men, evening exercise was the clear winner in terms of health. The evening exercisers significantly reduced their cholesterol levels, while the morning exercisers, surprisingly, slightly increased theirs. Evening exercise also boosted fat burning in men. At the end of the study, the bodies of men who exercised in the evening burned about 28% more fat during workouts than at the start, a change that may lead to body fat loss. The morning group’s fat burning increased only slightly.
- However, any time was the right time for men to improve their strength and fitness. Among men, those who trained in the morning and evening increased their bench press, leg press, sit-ups, push-ups, and other strength records to about the same extent, whether they had exercise sooner or later.
What these results mean in practical terms is that women with specific health or fitness goals may want to fine-tune their workout schedule, Arciero said. If you’re a woman hoping to lose inches in the middle, consider working out in the morning. If your goal is strength, evening workouts might be more effective.
For men, exercising sooner or later seems comparable for strength and fitness, but exercising at night might have particular health benefits, Arciero said.
Still, “it’s early days in terms of providing individualized prescriptions for the optimal time of day to exercise,” said John Hawley, head of the exercise and nutrition research program at the Australian Catholic University of Melbourne, Australia, which has extensively studied metabolism and timing of exercise, but was not involved in this study.
He pointed out that the new study did not control for women’s menstrual cycles or track people’s chronotypes – whether naturally morning or evening – which could both influence responses to exercise. It also did not include midday exercise or investigate why men and women reacted so differently to exercise time. Arciero suspects hormone and other cellular and genetic effects, and plans follow-up studies to learn more, he said.
For now, the key point of the study is that timing can fine-tune what we gain from exercise. But we benefit regardless, so “any time of day you choose to exercise is a good time,” Hawley said.
Do you have a fitness question? E-mail YourMove@washpost.com and we may answer your question in a future column.
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