You might think that a late workout helps you unwind before bed. Maybe you’ve even felt a post-gym crash that made you doze off faster. But new research suggests your body may be paying a hidden cost. In short, these workouts could chip away at your sleep quality and undermine the very recovery you’re aiming for.

A massive international study, published in Nature Communications, tracked nearly 15,000 people across more than four million nights of sleep. Turns out, late-night exercise (especially intense exercise) throws your system into overdrive. It delays sleep, shortens its duration, and weakens its restorative power.

The Four-Hour Rule

This has been discussed before, says Josh Leota, a sleep scientist at Monash University and the study’s lead author.

“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. However, findings from controlled laboratory studies are less conclusive, with many suggesting that evening exercise doesn’t necessarily disrupt sleep,” he said.

Leota and his team wanted clarity. So, they turned to the real world — and to data on a scale that’s rarely available in sleep research. Using wearable biometric devices, the researchers monitored the exercise and sleep habits of 14,689 physically active adults over the course of a year. This yielded more than 4 million person-nights of data.

What they found was conclusive: when workouts ended less than four hours before bed, participants had notably worse sleep across multiple dimensions. Results were adjusted for gender, age, weekday, season, general fitness and the prior night’s sleep, but the findings were still consistent, says Elise Facer-Childs, from the Monash University School of Psychological Sciences.

“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,” Dr. Facer-Childs said.

How big is the impact

If someone finished a high-strain workout just two hours before trying to sleep, their sleep onset was delayed by an average of 36 minutes. Sleep duration dropped by up to 13.9% and sleep quality by over 5% after late high-strain workouts. These weren’t minor disruptions: they affected people’s ability to fall asleep, stay asleep, and get the deep rest their bodies needed to recover. The closer the workout was to bedtime, the bigger the impact.

Heart rate data added another layer of insight. Late, strenuous workouts raised resting heart rate and lowered heart rate variability (HRV) — a marker of how well the autonomic nervous system recovers during sleep. Lower HRV at night has been linked to poorer cardiovascular recovery.

But not all evening workouts are equal, researchers found. The study divided exercise into four levels of strain — light, moderate, high, and maximal — based on cardiovascular load and duration.

Light exercise, like a brisk walk or gentle swim, had little to no effect on sleep when performed within that four-hour window. In fact, if done earlier in the evening, it could even correlate with slightly better sleep outcomes. The real culprit was high or maximal strain activity. Think football matches, HIIT circuits, or long-distance runs.

When you exercise hard, your sympathetic nervous system — the “fight or flight” side — kicks into gear. This leads to elevated heart rate, body temperature, and alertness. For good sleep to happen, your body needs to shift into parasympathetic mode, a calmer, restorative state. But if you wrap up a grueling gym session too late, that shift doesn’t happen fast enough. Instead of winding down, your body is still riding the exercise high.

What you should do

If you’re a “night owl athlete,” you should consider these findings closely. If you must exercise late, Leota advises sticking to lower-intensity options. A light run or swim, some yoga perhaps, nothing that will get your heart racing.

If you’re looking to optimize both fitness and sleep, here’s the takeaway: Give yourself at least four hours between the end of your workout and your usual bedtime. And, remember: sleep isn’t just downtime. It’s when your body repairs, grows, and recalibrates. Undermining that process with late-night workouts could counteract the very benefits you’re chasing.

We all know exercise is good for us. It boosts mood, supports metabolism, improves cardiovascular health, and yes — usually helps us sleep better. But, like anything else, context matters.

In recent years, the conversation around “chronofitness” — the study of how workout timing interacts with your biological rhythms — has grown louder. This new research adds serious weight to that discussion.

This study is a reminder that when and how you move also matters a lot. So, next time you’re tempted to squeeze in a hardcore workout at 9 p.m., ask yourself what you might be losing in return.

The study “Dose-response relationship between evening exercise and sleep” was published in the journal Nature Communications.