Exercise is good for you no matter the time of day, but a new randomized control trial has found breaking a sweat in the morning and the evening might target different parts of the body and mind.
Over the course of 12 weeks, 27 healthy and active women and 20 healthy and active men participated in a strict diet and training program.
The weekly routine involved four days of exercise, including sprints, resistance training, stretching and endurance training, plus three days of rest on Wednesday, Saturday, and Sunday.
Half the group conducted their hour-long bouts of daily exercise in the morning before breakfast, while the other half completed their routine before dinner in the evening.
All the participants ultimately showed significant improvements in their fitness and health, but morning and evening exercises had different outcomes, especially for women.
Female participants who exercised in the morning burned 7 percent more abdominal fat and reduced their blood pressure by 7 percent more than women who exercised in the evening. The morning routine also resulted in greater leg strength.
Women who exercised in the evening, on the other hand, showed greater gains in their upper body strength, their mood, and their food cravings. What's more, muscle power improved by 29 percent and endurance improved by 15 percent compared to morning exercisers.
Compared to women, men in the trial were generally less influenced by the time of exercise. That said, evening training led to slightly lower blood pressure and increased fat oxidation compared to morning exercise. Evening training also improved fatigue by 55 percent.
"Based on our findings, women interested in reducing belly fat and blood pressure, while at the same time increasing leg muscle power, should consider exercising in the morning. However, women interested in gaining upper body muscle strength, power and endurance, as well as improving overall mood state and food intake, evening exercise is the preferred choice," explains physiologist Paul Arciero from Skidmore College.
"Conversely, evening exercise is ideal for men interested in improving heart and metabolic health, as well as emotional wellbeing."
The study is the first to explore how a diverse exercise regime impacts individuals depending on what time of day the workout takes place.
Past research has also found that morning exercise holds different physical outcomes compared to afternoon or evening exercise, but data on diverse exercise routines are scarce and most studies have focused on males only.
Today, some estimates suggest female participants have been included in only 3 percent of all sports science research, and animal studies are often no better.
Previous studies on mice, for instance, have found that morning exercise contributes more to fat loss, while evening exercise holds greater control over blood-sugar levels. Yet this research only focused on male mice that performed one bout of aerobic exercise.
The new long-term trial includes both men and women, although its sample size is limited in other ways. Almost all the participants were Caucasian and physically fit.
Despite these limitations, the findings suggest the timing of daily exercise impacts female physical performance to a greater degree than male physical performance.
Why that is remains undetermined, but the authors have a few hypotheses. Previous studies have shown that men and women have different circadian rhythms, which impact a person's physiology and psychology throughout the day.
In fact, every single cell in the human body is in tune to its own clock, cycling through patterns of activity on a roughly 24-hour basis.
Timing exercise so that it coincides with certain peaks and pits in hormonal levels, metabolism and neuromuscular factors could, theoretically, influence a person's muscle strength, their cardiovascular system, their body composition and their physical performance.
The authors suspect, for example, that a night of fasting somehow primes the female body for greater fat loss in the morning.
On the other hand, men exercising in the evening are working with a metabolism at its peak. This may provide an advantage when it comes to using body fat as fuel for evening workouts.
While losses in body fat were similar in male participants no matter the time of exercise, those who worked out in the evening showed increased fat oxidation, which could be a sign that the body is preparing for actual fat loss in the long-term.
Training sessions that go longer than 12 weeks could help determine if that truly is the case.
The perfect time for exercise is still hotly debated, but more diverse, long-term trials like the current one could help clear up the conflicting data scientists have gathered so far.
The study was published in Frontiers in Physiology.