Ever knock out a tough evening interval session and then lie in bed staring at the ceiling? You’re not alone. While breaking a sweat is great for your health—and your mood—there may be a cutoff point if you care about quality sleep. A massive new study published in Nature Communications shows that running (or any intense exercise) too close to bedtime might mess with your body’s ability to power down. Here’s what that means for your training schedule—and your pillow time.
The four-hour rule
Researchers tracked 14,689 active adults over a year, collecting more than four million nights of sleep and exercise data via wearable biometric devices. They weren’t just looking at bedtime or step count—they examined when people worked out, how hard they went and how that affected their rest later that night.
The findings? When participants finished high-strain workouts, like long runs, HIIT or football, less than four hours before their usual sleep time, they experienced noticeable disruptions. We’re talking later sleep onset, shorter sleep duration, poorer sleep quality and elevated resting heart rates overnight. Their nervous systems were still buzzing.
Lighten up to wind down
Interestingly, it wasn’t just about the clock—it was about the intensity. Light or moderate workouts (like an easy run or yoga flow) that wrapped up at least two hours before sleep didn’t cause the same issues. These gentler sessions had sleep outcomes similar to rest days.
But once the intensity ratcheted up, especially closer to bedtime, so did the body’s stress response. Higher heart rates and lower heart rate variability hours after exercise suggest the body is still in recovery mode, making it harder to slip into that crucial parasympathetic state where real rest happens.
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Why this matters for runners
Sleep isn’t just recovery—it’s also where the gains happen. Poor sleep can impact everything from muscle repair to decision-making on the trail. This study suggests that runners pushing hard in the evening may be compromising one of the most important tools in their training toolbox.
If evening workouts are your only option, consider easing the effort—skip the speedwork and go for a relaxed cruise instead. Better yet, aim to finish your run at least four hours before you plan to hit the sack.
