A new study of nearly 15,000 people found that the window of late-night workouts in which sleep is then impacted is actually much larger than previously thought, showing that exercising within four hours of bedtime can have a detrimental effect on shut-eye.
Monash University researchers looked at international data from 14,689 people who were monitored with a multi-sensor biometric gadget for a year, which equaled four million nights of information to analyze. The tracking device, a WHOOP Strap, recorded exercise data, sleep and cardiovascular activity.
But the scientists weren’t looking at the impact of any exercise; they were more interested in how more intense workouts might have enduring heart-rate variabilities throughout the night. What they found was a link between night exercise and delayed sleep onset and less sleep achieved throughout the night, as well as a higher resting heart rate than seen in those who worked out earlier in the day.
“Intense exercise in the evening can keep the body in a heightened state of alertness, which is why public health guidelines have previously advised against working out too close to bedtime,” said Josh Leota, from the Monash University School of Psychological Sciences. “However, findings from controlled laboratory studies are less conclusive, with many suggesting that evening exercise doesn’t necessarily disrupt sleep.
“These studies have relied on small sample sizes and laboratory settings, and rarely involved exercise bouts that elicit substantial cardiometabolic demand on the body, calling into question the external validity of such findings,” he added.
Leigh Henningham
Previous studies have tied late-night workouts to sleep disturbance, but the exact time frame has varied. In 2021, Concordia University found that the switch from helpful to hindrance was exercise within two hours of bedtime.
The Monash team adjusted the data to account for factors such as age, gender, weekday, season, overall participant fitness and how they’d slept the night before without evening exercise.
Now, most of us know that there are enduring metabolic impacts of intensive activities like long runs, high-intensity impact training (HIIT) or even sports training sessions – which often occur at night after work. Those effects include elevated core body temperature and heart rate and mental alertness. But the team found that previous estimates of exercise-related sleep disturbances might have underestimated the window of time you have to avoid nocturnal unrest – by several hours.
“Evening exercise – particularly involving high levels of cardiovascular strain – may disrupt subsequent sleep, resting heart rate, and heart rate variability, thereby impairing a critical stage of the recovery process,” said senior author Elise Facer-Childs, from the Monash University School of Psychological Sciences.
She added that while it doesn’t prove causation, the team’s results found a consistent relationship across participants even when their data had been adjusted to factor in variables mentioned above.
“Our novel and timely findings have significant implications for public health messaging around timing, duration and intensity of exercise and present a critical step towards improving population sleep health,” Facer-Childs added. “One in five adults fail to achieve the recommended seven or more hours of sleep per night.”
Of course, many of us exercise in the evening for various reasons – such as work and family commitments. And it’s still a healthy way to de-stress and maintain fitness if any other time is prohibitive. However, the researchers suggest being mindful that the “when” and “what” is key to giving you the best chance at a good night’s sleep. What’s more, earlier studies found that poor sleep could negate the benefits to brain health gained through exercise.
“If exercising within a four-hour window of bedtime, people could choose brief low intensity exercises, such as a light jog or swim, to minimize sleep disruption and allow the body to wind down,” said Leota.
And those who enjoy evening exercise sessions may also benefit from monitoring their overnight biometrics with a tracker, just to see if any sleep issues could be linked to late-night workouts, and then adjusting the intensity to see if they could be related. Of course, any exercise is better than no exercise – but the scientists say being aware of how it could impact sleep can help people get the best outcomes for their bodies overall.
The study was published in Nature Communications.
Source: Monash University