What is the best time of day to exercise?

It’s a straightforward question with a frustrating number of answers, based on research results that can be downright contradictory.

The latest piece of evidence came last month from a group of Australian researchers, who argued that evening was the healthiest time to break a sweat, at least for those who are overweight. Their study looked at 30,000 middle-aged people with obesity and found that evening exercisers were 28% less likely to die of any cause than those who worked out in the morning or afternoon.

“We were surprised by the gap,” said Angelo Sabag, an exercise physiologist at the University of Sydney who led the study. The team expected to see a benefit from evening workouts, but “we didn’t think the risk reduction would be as pronounced as it was.”

So does that mean that evening swimmers and night runners had the right idea all along?

“It’s not settled,” said Juleen Zierath, a physiologist at the Karolinska Institute in Sweden. “It’s an emerging area of research. We haven’t done all the experiments. We’re learning a lot every month.”

No single study can dictate when you should exercise. For many people, the choice comes down to fitness goals, work schedules and plain old preferences. That said, certain times of day may offer slight advantages, depending on what you hope to achieve.

The case for morning exercise

According to a 2022 study, morning exercise may be especially beneficial for heart health. It may also lead to better sleep.

And when it comes to weight loss, there have been good arguments made for morning workouts. Last year, a study published in the journal Obesity found that people who exercised between 7 and 9 a.m. had a lower body mass index than counterparts who exercised in the afternoon or at night, although it did not track them over time, unlike the Australian study, which followed participants for an average of eight years.

Of course, the biggest argument for morning exercise may be purely practical.

“For a lot of people, the morning is more convenient,” said Shawn Youngstedt, an exercise science professor at Arizona State University. Even if rising early to work out can be challenging at first, morning exercise won’t get in the way of Zoom meetings, play dates or your latest Netflix binge.

The case for afternoon exercise

A few small studies suggest that the best workout time, at least for elite athletes, might be the least convenient for many of us.

Body temperature, which is lower in the morning but peaks in late afternoon, plays a role in athletic performance. Several recent small studies with competitive athletes suggest that lower body temperature reduces performance (although warm-up exercises help counter that) and afternoon workouts help them play better and sleep longer.

If you have the luxury of ample time, one small New Zealand study found that it could help to nap first. As far as the rest of us are concerned, a Chinese study of 92,000 people found that the best time to exercise for your heart was between 11 a.m. and 5 p.m.

“The main difference is our population,” Sabag said. While his study was restricted to obese people, the Chinese study was not. “Individuals with obesity may be more sensitive to the time-of-day effects of exercise.”

The case for evening exercise

This latest study may not settle the debate, but it certainly suggests that those struggling with obesity might benefit from a later workout.

Exercise makes insulin more effective at lowering blood sugar levels, which in turn fends off weight gain and Type 2 diabetes, a common and devastating consequence of obesity.

“In the evening, you are most insulin resistant,” Sabag said. “So if you can compensate for that natural change in insulin sensitivity by doing exercise,” he explained, you can lower your blood glucose levels, and thus help keep diabetes and cardiovascular disease at bay.

One persistent concern about evening exercise is that vigorous activity can disturb sleep. However, some experts have argued that these concerns have been overstated.

The case that it may not matter

While many of these studies are fascinating, none of them are definitive. For one thing, most are simply showing a correlation between exercise times and health benefits, not identifying them as the cause.

“The definitive study would be to actually randomize people to different times,” Youngstedt said, which would be phenomenally expensive and difficult for academics.

One thing public health experts do agree on is that most Americans are far too sedentary. And that any movement is good movement.

“Whenever you can exercise,” Sabag urged. “That is the answer.”

In a recent edition of his newsletter that discussed the Australian study, Arnold Schwarzenegger — bodybuilder, actor, former governor — seemed to agree. He cited a 2023 study suggesting that there really isn’t any difference in outcomes based on which time of day you exercise. In which case, it’s all about what works best for you.

“I will continue to train in the morning,” wrote Schwarzenegger, a former Mr. Universe. “It’s automatic for me.”