NEW YORK, May 12: Exercise performed too close to bedtime could negatively affect sleep quality, timing, and duration, according to a new study led by Monash University. The research, published in Nature Communications, revealed that more intense workouts near bedtime are associated with greater disruptions in sleep and nighttime cardiac activity.

The study, which is the largest of its kind, discovered that exercising within four hours of bedtime led to later sleep onset, shorter sleep duration, poorer sleep quality, and higher resting heart rate coupled with lower heart rate variability. This study is the first to identify these specific associations.

The research involved an international sample of 14,689 participants, with data collected over the span of one year, resulting in an analysis of four million nights of data. Participants wore multi-sensor biometric devices (WHOOP Strap) that recorded exercise, sleep, and cardiovascular data.

Monash University and WHOOP researchers examined how evening exercise, the strain of the workout, and nocturnal cardiac activity—including resting heart rate and heart rate variability—were related. They found that the combination of later exercise timing and higher exercise intensity led to delayed sleep onset, shorter sleep duration, poorer sleep quality, higher nocturnal resting heart rate, and lower nocturnal heart rate variability.

The results were adjusted for factors such as gender, age, weekday, season, general fitness, and the previous night’s sleep. High-strain exercises include activities that significantly increase heart rate, breathing rate, body temperature, and mental alertness, such as HIIT (high-intensity interval training), football or rugby games, and long-distance running.

Dr. Josh Leota, the study's lead author from Monash University's School of Psychological Sciences, emphasized the importance of understanding the link between exercise timing and sleep. He noted that intense exercise in the evening keeps the body in a heightened state of alertness, which may explain why public health guidelines have advised against late-night workouts. However, Dr. Leota pointed out that controlled laboratory studies have had mixed results, with some suggesting that evening exercise does not always disrupt sleep. He also noted that many of these studies involved small sample sizes and exercise regimens that did not place significant cardiometabolic demands on the body, questioning the validity of their findings.

Dr. Leota suggested that if people wish to improve sleep health, they should aim to finish their exercise routine at least four hours before bedtime. For those who need to exercise closer to bedtime, low-intensity activities like light jogging or swimming may help minimize sleep disruption and allow the body to wind down.

Dr. Elise Facer-Childs, a senior author on the study from Monash University, commented that the findings show a clear, consistent relationship between evening exercise, sleep quality, and nocturnal cardiac function. "Evening exercise—especially when it involves high levels of cardiovascular strain—can disrupt sleep, resting heart rate, and heart rate variability, all of which impair the body's recovery process," she explained.

Dr. Facer-Childs highlighted the importance of the findings for public health messaging. "Our research has significant implications for how we communicate the timing, duration, and intensity of exercise. Given that two-thirds of Australian adults report sleep problems, and one in five adults fail to get the recommended seven or more hours of sleep per night, these insights are crucial for improving population sleep health."

The study underscores the importance of balancing exercise routines with sleep needs, suggesting that proper timing and intensity of workouts could play a critical role in improving overall health and well-being.