Study details

For the study, which was published in Nature Communications, researchers from Monash University analyzed anonymized biometric data from 14,689 adults who wore a Whoop activity tracker for at least a year. The dataset included more than 4 million nights of sleep and corresponding exercise sessions, categorized by timing and intensity.

The researchers assessed the relationship between exercise strain — defined by workout intensity and duration — and sleep outcomes, including onset sleep latency, total sleep time, sleep quality, resting heart rate, and heart rate variability.

The researchers found that workouts ending within four hours of bedtime, particularly those with high strain, were consistently associated with delayed sleep and reduced overnight recovery.

The researchers said they believe the link between late workouts and poor sleep quality is largely physiological.

Intense exercise increases sympathetic nervous system activity, raising heart rate, core body temperature, and mental alertness. These effects can persist for hours, especially after high-strain workouts, leaving the body in an activated state and unable to attain restful sleep.

Tracker data confirmed that strenuous evening exercise significantly elevated nocturnal resting heart rates and reduced heart rate variability (HRV). In some cases, heart rates remained elevated well into the night, while HRV — a key indicator of parasympathetic nervous system recovery — remained stubbornly low.

Both metrics suggest delayed autonomic recovery, meaning the body stays in a "revved up" state that makes it harder to fall and stay asleep.

Discussion

The study was observational and therefore unable to prove that late workouts directly cause poor sleep. Participants were also all Whoop users, meaning that their sleeping habits and behaviors may differ from the general population. The analysis also did not account for other factors that can affect sleep, including late meals, bright light exposure, or screen use.

Even with these limitations, experts say that the study offers valuable real-world insights. Kenneth Wright Jr., a professor of integrative physiology at the University of Colorado at Boulder who was not involved in the study, said the findings reinforce the role of behavior in sleep health.

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

And if you do need to exercise late at night, Leota recommends trying meditation, gentle yoga, or other relaxation techniques afterwards to calm yourself down.

"We are definitely not discouraging exercise," he 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."