Does it make a difference what time of day you exercise? Several studies suggest that it does. But it’s complicated.

Morning exercise may activate certain genes in the muscle cells, boosting their ability to metabolize sugar and fat, helping those who are overweight or suffering from Type 2 diabetes.

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 compared to the morning, as [athletes] consume less oxygen, that is, they use less energy, for the same intensity of exercise in the evening vs. the morning,” said 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 said.

A second study led by Paolo Sassone-Corsi, director of the Center for Epigenetics and Metabolism at the University of California at Irvine, took a different approach. It looked at the changes in muscle tissues after morning workouts, specifically the glucose breakdown and fat burning. In analyzing the tissue, researchers found that exercise seemed to provide the most beneficial effects on metabolism during late morning.

“We identified that time of exercise is critical in order for exercise to be beneficial” in metabolizing sugar and fat, he said.

Sassone-Corsi believes the difference in the body’s reactions has to do with its circadian clock, the internal mechanism that influences human cycles of sleep, awakening and eating, among other things.

“Circadian rhythms dominate everything we do,” he said. “At least 50% of our metabolism is circadian. The metabolic cycles are not adapted to respond to external stimuli the same way at day or night. “

Moreover, 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 energize 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,” said Edward R. 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, said that the what of exercising should take precedence over the when.

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