Australians hitting the gym in the evening may be short-changing their sleep, according to new research from Monash University that tracked nearly 15,000 people worldwide over a year. The findings suggest intense exercise less than four hours before bedtime is linked with shorter and poorer quality sleep—and a heart that doesn’t fully wind down overnight.
Published in Nature Communications, the research team, working with wearable tech company WHOOP, analysed four million nights of sleep data collected from participants wearing WHOOP’s biometric tracking device. The study is the largest of its kind to date and is the first to establish a clear association between late-evening workouts and measurable changes in sleep and overnight cardiac patterns.
The lead author, Dr Josh Leota from the Monash University School of Psychological Sciences, said the results address a long-standing debate about whether exercising close to bedtime interferes with sleep. “Public health advice has often warned against it,” he noted, “but until now, evidence has been mixed—largely due to smaller sample sizes or artificial lab conditions.”
The study found that high-intensity workouts—such as HIIT, long-distance running or competitive team sports—performed within four hours of going to bed led to delayed sleep onset, reduced total sleep, poorer sleep quality, higher resting heart rates and lower heart rate variability. These latter two indicators are linked to the body’s ability to recover during rest and are essential for overall cardiovascular health.
Co-author Dr Elise Facer-Childs said the results were strikingly consistent, even after adjusting for a wide range of variables including age, gender, fitness levels, seasons, and the prior night’s sleep. “The more intense and later the exercise, the more disruption we observed across every sleep and cardiac measure,” she said.
Dr Leota believes the problem lies in how the body responds to strenuous activity. “Exercise raises body temperature, boosts mental alertness, and activates the sympathetic nervous system—what we call the ‘fight or flight’ response,” he said. “If that activation carries into the night, it becomes much harder to transition into restful sleep.”
Interestingly, the study found no such disruption when workouts were completed earlier in the day or when evening exercise was kept at a low intensity, such as a light walk or gentle swim.
The findings may carry weight for Australia’s broader sleep problem. According to government data, two in three adults report at least one issue with their sleep, and one in five fail to get the recommended seven hours per night. With cost-of-living pressures pushing more people to exercise late—after work or after dinner—the Monash team believes timing needs to be part of the conversation.
While previous lab studies didn’t detect much of an effect, Dr Leota says that’s because they rarely replicated real-life workouts. “A treadmill stroll in a controlled setting can’t mimic the cardiovascular stress of a real football match or an intense interval session. We wanted to understand what actually happens in the real world.”
The researchers suggest anyone looking to improve sleep might benefit from finishing workouts at least four hours before bed. If evening is the only time available, they recommend opting for shorter, low-strain activities to reduce the chances of disrupting rest.
The findings also add a fresh layer to public health messaging on sleep and fitness—one that goes beyond simply telling people to exercise and instead invites consideration of how and when. “It’s not just about doing exercise—it’s about doing it in a way that supports the whole recovery process,” Dr Facer-Childs said.
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