But there may be ways to minimise the effects if evening happens to be the only time you can – or care to – work out.

The link between exercise and sleep

For decades, researchers have been puzzled by the relationship between sleep and exercise. According to most past research, active people sleep better than the sedentary, but not always. Some studies suggest morning workouts improve sleep, while later workouts don’t, but others seem to show any movement, at any time, helps people nod off earlier.

Most of these studies have been quite small, though, often involving fewer than 20 volunteers, and relied on people’s memories of when and how they worked out and snoozed.

So for the new study, published in April in Nature Communications, researchers at Monash teamed up with the activity-tracker maker Whoop to parse anonymised data from 14,689 men and women aged 18 to 87 who’d worn a Whoop tracker for at least a year. (Whoop provided access to the data but “did not have any input into the analysis or results,” Leota said.)

The records included extensive details about when and how intensely people exercised every day, based on their heart rates, and also how well they’d slept that night, including when they’d nodded off, how long they’d remained asleep and the overall quality of their slumber.

Thirty-six extra minutes to fall asleep

The researchers were interested in how late-day exercise changes sleep – since previous studies had so often disagreed with one another. They first categorised people’s workouts as light, moderate, hard or maximal, corresponding, in broad terms, to a brisk walk, easy jog, long run or prolonged high-intensity interval training. They also took note of when people worked out and mapped their sleep.

Then they cross-checked. Did people sleep better or worse after they worked out close to bedtime? What if the exercise was gentle? What if they pushed themselves?

The answers consistently showed that “later exercise timing and higher exercise strain” were each strongly linked to worse sleep, the scientists wrote in the study. Even relatively modest evening workouts, such as light weight training or a gentle gym class, could somewhat disrupt sleep.

But the impacts intensified with the intensity. If people ran an after-hours half-marathon or played a rousing late-night soccer, hockey or basketball game within about two hours of their usual bedtime, they needed an average of 36 extra minutes to fall asleep.

Finish that same strenuous exercise even later at night, after someone’s usual bedtime by an hour or two, and he or she would need an extra 80 minutes to doze off.

People also slept less, in total, after hard, evening exercise, and the quality of their sleep declined, with frequent waking, tossing and turning.

How to wind down after a late workout

The researchers didn’t look at why this happens, but they suspect people were too wound up, physiologically. Participants’ tracker data showed their heart rates were still elevated hours after strenuous evening exercise, while, at the same time, their heart rate variability, which should be somewhat high, remained stubbornly low.

In essence, Leota, said, people got too pumped up by vigorous, late-night workouts to easily drift off or stay asleep. “A basic rule of thumb,” he said, “is the harder you work out, the more time you need to give yourself to recover before going to sleep.”

If you do need to exercise late in the evening, you might want to try meditation, gentle yoga or other relaxation techniques afterward to calm your revved-up body, Leota said.

Even better, “if you can exercise earlier in the day, that would be preferable,” he said.

But if the evening is your best option, stick with it. “We are definitely not discouraging exercise,” Leota said. “For the vast majority of people, any exercise is better than no exercise. We would just recommend trying to finish as early as possible or opting for lighter workouts.”

This study has limitations. It’s associational, showing links between evening exercise and sleep disruption, but not that one directly causes the other. It also can’t tell us whether it was evening exercise or other factors – like late dinners and exposure to bright lights – that most disrupted people’s sleep. Plus, the participants had all bought and worn an activity tracker, meaning they might not be representative of the many people who don’t track their lives, sleep and movements.

Even with these caveats, though, the results resonate, said Kenneth Wright jnr, a professor of integrative physiology at the University of Colorado at Boulder, who studies sleep and physical activity, but wasn’t involved with the new study.

“We need more studies like this one to assess the impacts of environmental and behavioural factors on sleep health,” he said. “The findings provide support that to promote sleep health, a general recommendation to avoid intense exercise within four hours of bedtime is warranted.”