It found that for women, morning workouts reduce belly fat and improve blood pressure better than workouts later in the day. For men, evening exercise increased fat burning and improved blood pressure control. Evening exercise also amplified the benefits of strength training, but more so for women.
Exercise timing studies are part of the burgeoning science of chronobiology, which focuses on how our internal clocks affect nearly every aspect of our physiology.
Humans’ bodies, like those of mammals, plants, reptiles, and other insects, operate in an innate 24-hour circadian system, with a master clock system in our brains sending and receiving biochemical signals that coordinate with molecular clocks within our cells to direct something bewildering. 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 have allowed large groups of rodents to run on exercise wheels at different times of the day. Studies have shown that animals’ heart rate, fat burning, gene expression, and body weights change dramatically, depending on when they exercise — even if the exercise itself is the same.
Human studies on the timing of exercise have been more contradictory. Some show that people burn extra fat and lose more weight by exercising earlier, especially before breakfast, while others suggest we get greater health benefits from afternoon or evening workouts.
But most of these studies were small and only included men with metabolic diseases, such as type 2 diabetes, high blood pressure or obesity. So we didn’t know much about the optimal workout timing for healthy men—and even less about the best timing for women. This is why the new study is so important.
Realistic study of exercise timing
The research was designed to reflect real-world demographics, said Paul Arceiro, director of the Laboratory of Human Nutrition, Performance, and Metabolism at Skidmore College, in Saratoga Springs, New York, and lead author of the study.
All volunteers identified themselves as either male or female, and more than half of the 56 participants were women. They were all healthy and physically active but not very athletic.
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., and the other group was instructed to exercise in the evening, between 6:30 and 8:30 p.m.
Each group participated in a similar exercise. Once a week, they lift weights. The next day, they did about 35 minutes of interval training (running, swimming or cycling as hard as they could for about a minute, resting and repetitions). Another day, do yoga or pilates. Finish the week with about an hour of running, biking, or other aerobic exercise.
The groups maintained this routine for 12 weeks, then returned to the lab for retesting.
Everyone was in the study leaner, faster, fitter, stronger, healthier, and more flexible, whether they’re working out early or late.
Do you want to get rid of belly fat? Or do we build strength?
But there were relevant differences between the groups based on the time of day they exercised. Here’s what the researchers found:
- For women, fat is burned best in the morning. The percentage of total body fat in women who exercised early fell, on average, about 3 percent more than those who exercised in the evening, with a significant loss of their waist circumference. The morning exercisers shed 7 percent more fat than the women who exercised in the evening. (The volunteers’ total body weight did not decrease, because they gained muscle because they lost fat.)
- Also, morning exercises lower blood pressure in females much better than the same exercises in the evening.
- Meanwhile, the women’s evening exercise amplified strength gains. Overall, the evening exercisers improved their upper body strength 7 percent more than the morning group, and they also did more sit-ups and push-ups.
- For men, evening exercise was the clear winner in terms of health. Evening exercisers significantly lowered their cholesterol levels, while morning exercisers raised their levels slightly. Evening exercise increased fat burning in men. By the end of the study, men who exercised in the evening were burning about 28 percent more fat during their workouts than they did at the beginning, a shift that can lead to a loss of body fat. Fat burning in the morning group grew only slightly.
- However, any time was the right time for men to increase their strength and fitness. Among the men, the morning and evening exercisers raised bench presses, leg presses, sit-ups, push-ups, and other strength roughly the same amount, whether they exercised early or later.
In practical terms, Archero said, what these results mean is that women with certain health or fitness goals may want to adjust the timing of their workouts. If you’re a woman hoping to lose inches around the middle, consider morning workouts. If your goal is strength, evening workouts may be more effective.
For men, exercising earlier or later seems comparable in terms of strength and fitness, but evening exercise can have special benefits for health, Arceiro says.
John Hawley, head of the Exercise and Nutrition Research Program at Australian Catholic University in Melbourne, Australia, which has extensively studied exercise and timing but was not involved in this study, said.
He noted that the new study did not control women’s menstrual cycles or track people’s temporal patterns — whether they were in the morning or evening — both of which can influence exercise responses. Nor did it include midday exercises or consider why men and women react differently to the timing of exercise. He said Arcero suspects hormones and other cellular and genetic influences, and is planning follow-up studies to find out more.
For now, the main takeaway of the study is that timing may fine-tune what we gain from exercise. But we benefit, regardless, that “any time of day you choose to exercise is the right time,” Hawley said.
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