An evening workout, on the other hand, uses less oxygen, making workouts more efficient and improving athletic performance, potentially a boon for serious competitors.

“Human exercise performance is better in the evening than the morning, as (athletes) consume less oxygen, that is, they use less energy, for the same intensity of exercise in the evening versus the morning,” says Gad Asher, a researcher in the Weizmann Institute of Science’s department of biomolecular sciences, and author of one of the studies.

“It means, for example, if a person needs to go for a run, he will reach exhaustion earlier in the morning compared to the evening,” Asher says. “In other words, he will be able to run for a longer duration in the evening compared to the morning under the same running conditions.”

Asher’s group put mice on treadmills at different times of day and studied their exercise capacity at different intensities and regimens. They found that overall exercise performance was vastly better – about 50 per cent on average – during the “mouse evening”, compared to the morning hours. They also studied 12 humans and saw similar results. Overall, the human subjects consumed less oxygen while exercising in the evening, compared with morning.

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A second group led by Paolo Sassone-Corsi, director of the Centre for Epigenetics and Metabolism at the University of California at Irvine, also put mice on treadmills, but took a different approach.

The researchers looked at the changes in muscle tissues after morning workouts, specifically the glucose breakdown and fat burning. In analysing the tissue, they found that exercise seemed to provide the most beneficial effects on metabolism during the mouse equivalent of what would be late morning for humans.

“We identified that time of exercise is critical in order for exercise to be beneficial” in metabolising sugar and fat, Sassone-Corsi says.

Sassone-Corsi believes this is controlled by a process that relies on a specific protein, HIF1-alpha, which directly regulates the body’s circadian clock, the internal mechanism that influences human cycles of sleep, awakening and eating, among other things.

“Circadian rhythms dominate everything we do,” Sassone-Corsi says. “At least 50 per cent of our metabolism is circadian, and 50 per cent of the metabolites in our body oscillate based on the circadian cycle. It makes sense that exercise would be one of the things that’s affected.

“There is a time for exercise, resting or food intake,” he adds. “The metabolic cycles are not adapted to respond to external stimuli the same way at day or night.”

So which is the better time to exercise – morning or evening? It depends on your goals.

Elite and otherwise serious athletes – marathon runners, basketball and soccer players seeking a competitive edge, for example – might choose evenings to train or compete. Similarly, those who schedule important sports events might consider holding them at night to ensure optimal performances.

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“If you wish to break the world record, or your personal time, I assume (evenings would be better),” Asher says.

Those who worry more about their weight and controlling their blood sugar – and less about shaving a minute or two from their marathon time – might go for mornings, when post-workout cell responses that influence metabolism are much stronger.

Jonas Thue Treebak, associate professor at the Novo Nordisk Foundation’s Centre for Basic Metabolic Research at the University of Copenhagen and co-author of a third study, agrees.

“At this point, we can only conclude that the effects of the two appear to differ, and we certainly have to do more work,” he says.

There are other things beyond performance and weight loss to take into account.

“Exercising late at night may interfere with sleep as it tends to energise you and enhance alertness, although some people like to exercise at the end of the day to help relieve the stresses of the day and prepare for evening activities, which is fine,” says Edward Laskowski, co-director of the Mayo Clinic Sports Medicine and professor in the department of physical medicine and rehabilitation.

“Morning exercise has the advantage that no matter what else happens during the day, you have incorporated your physical activity. It also increases alertness and helps cognitive functioning.”

His Mayo Clinic colleague Michael Joyner, who studies how humans respond to different physical stresses, including exercise, agrees.

The research “tends to suggest that morning exercising before eating is helpful in terms of ensuring or maximising some of the positives effects of exercise on metabolism”, he says. “To me, the other positive of exercising first thing is that you get it done before the day catches up with you.”

But the most important thing – as a well-known athletic shoe company used to say in its advertising – is to just do it, regardless of when.

“The ‘do something’ message is far more important than the ‘do something at a specific time of the day’ message’,” Joyner says.