People who exercise in the morning seem to lose more weight than people completing the same workouts later in the day, according to a new study of workouts and waistlines. The findings help shed light on the vexing issue of why some people shed considerable weight with exercise and others almost none, and the study adds to the growing body of science suggesting that the timing of various activities, including exercise, could affect how those activities affect us.

The relationship between exercise and body weight is somewhat befuddling. Multiple past studies show that a majority of people who take up exercise to lose less weight drop fewer pounds than would be expected, given how many calories they are burning during their workouts. Some gain weight.

But a few respond quite well, shedding pound after pound with the same exercise regimen that prompts others to add inches.

This variability interests and puzzles Erik Willis, a data analyst with the Center for Health Promotion and Disease Prevention at the University of North Carolina at Chapel Hill. For almost a decade, he and colleagues at the University of Kansas, the University of Colorado Denver and other institutions have overseen the Midwest Exercise Trial 2, an extensive examination of how regular, supervised exercise influences body weight.

In that study, about 100 overweight, previously inactive young men and women worked out five times a week at a physiology lab, jogging or otherwise sweating until they had burned up to 600 calories per session.

After 10 months of this regimen, almost everyone had dropped pounds. But the extent of their losses fluctuated wildly, even though everyone was doing the same, supervised workouts.

When, for a 2015 study, the researchers tried to tease out what had distinguished the biggest losers from those who had lost less, they turned up surprisingly few differences. In line with other recent studies, they found that some participants, especially men, had begun eating more than before the study, but only by about 100 calories or so a day.

Flummoxed, Willis and one of his collaborators, Seth Creasy, a professor of exercise physiology at the University of Colorado Denver’s Anschutz campus, started brainstorming other possible, perhaps unexpected contributors to the enormous variability to weight loss.

They hit upon activity timing.

The science of chronobiology, which studies the ways in which when we do something alters how our bodies respond, is of great interest now. Many recent studies have looked at how meal timing, for instance, affects weight control, including whether exercising before or after breakfast matters. But far less has been known about whether the timing of exercise, by itself, influences whether people lose weight with workouts.

So, for the new study, which was published in July in The International Journal of Obesity, Willis and his colleagues sifted through their data again, this time looking at when people in the Midwest trial had shown up at the university lab.

In that study, participants could visit the gym whenever they wished between 7 a.m. and 7 p.m., signing in each time, so researchers had plenty of precise information about their schedules. The scientists also had tracked everyone’s calorie intakes and daily movement habits throughout the 10 months, using activity trackers and liquid energy tracers. They knew, too, whether and by how much people’s weights had changed.

Now, they checked weight change against exercise schedules and quickly noticed a consistent pattern.

Those people who usually worked out before noon had lost more weight, on average, than the men and women who typically exercised after 3 p.m. (For unknown reasons, very few people went to the gym between noon and 3.)

The researchers uncovered a few other, possibly relevant differences between the morning and late-day exercisers. The early-exercise group tended to be slightly more active throughout the day, taking more steps in total than those who worked out later. They also ate a bit less, although the difference amounted to barely 100 calories per day on average. Overall, such differences were barely discernible.

Yet, they may cumulatively have contributed to the striking differences in how many pounds people lost, Willis says.

Of course, this study was not large or designed from the start to delve into the chronobiology of exercise and weight. The researchers had not randomly assigned people to work out at particular times, so the links between exercise timing and weight loss they saw now in their reanalysis could be odd accidents related to individual participants’ preferences and schedules with little relevance for the rest of us.

Still, the statistical associations were strong, Willis says. “Based on this data, I would say that the timing of exercise might — just might — play a role” in whether and to what extent people drop pounds with exercise, he says.

But he also points out that most of those who worked out later in the day did lose weight, even if not as much as the larkish exercisers, and almost certainly became healthier.

“I would not want anyone to think that it’s not worth exercising if you can’t do it first thing in the morning,” he says. “Any exercise, at any time of day, is going to be better than none.”

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