Physiology Friday #268: Strenuous Workouts May Undermine Sleep-But Timing is Crucial

A new study of more than 14,000 WHOOP users finds that exercising too hard too close to bedtime might spell troubler.

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“Fatigue is the best pillow” – Benjamin Franklin

Over the past five years, a growing body of evidence has solidified what many of us already suspected (or have personally experienced): regular physical activity can significantly improve sleep.

Exercise enhances both how much we sleep (the quantity) and how well we sleep (the quality)—it helps people fall asleep faster, sleep longer, and spend more time in deep, restorative stages of sleep. These benefits are observed across a wide range of exercise types—including aerobic, resistance, and mind-body modalities and hold for people with sleep difficulties like insomnia. While the improvements are not usually dramatic (we’re talking on the order of minutes or small percentage points), they are meaningful, especially for anyone struggling with sleep or feeling unrested.

Importantly, these improvements aren’t just subjective. Objective sleep metrics like sleep efficiency and slow-wave sleep duration also trend upward with regular training. While a single workout can lead to modest gains in sleep on the same night, the biggest benefits emerge after several weeks, months, and years of consistent training.

There’s plenty of mechanistic evidence to support the sleep-promoting effects of regular exercise: it reduces stress and anxiety, promotes relaxation by enhancing parasympathetic nervous system activity, helps regulate body temperature to support sleep onset, and reinforces circadian rhythms. These biological and psychological effects combine to create a body and brain that are more “ready” for sleep after regular physical activity.

And let’s be honest, there’s also the fact that if you’re training hard (but not too hard), exercise can make you just plain old tired. A few hours of cardio per day will solve any sleep problems you have (trust me).

That’s as long as you get the timing (and intensity) right.

Despite old-school advice warning against evening workouts, newer analyses show that moderate-intensity evening exercise typically doesn’t disrupt sleep. But hard training or competition elevates core body temperature leads to high levels of sympathetic arousal—processes that actively oppose recovery-promoting sleep, suggesting that whether exercise is good or bad for sleep may depend on the context. The combination of high-intensity training close to bedtime remains a gray area—one where emerging data suggests sleep may still be compromised.

That’s where a new study published in Nature Communications enters the picture.¹

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Using over four million nights of biometric sleep and activity data, the study aimed to investigate how the timing and intensity of evening exercise interact to affect sleep and recovery. Most prior research on exercise and sleep has involved lab-based settings with short study durations and small participant pools and typically investigated only exercise intensity or timing in isolation.

This new study included 14,689 physically active adults and used biometric data from WHOOP wearable devices to track a full year of real-world activity and sleep. This scale and depth allowed the researchers to examine dose-response relationships—how different combinations of exercise timing and strain (WHOOPS’s proprietary score for exercise duration x intensity) impact not just whether sleep is disrupted, but how much.

Exercise strain was measured with a combined score called the Summated-Heart-Rate-Zones Score (SHRZS), a method that quantifies how long participants spent in different heart rate zones during a workout. Each minute in a higher zone contributes more to the total strain score, which was then categorized as:

• Light (<116 SHRZS)

• Moderate (116–214)

• High (214–461)

• Maximal (>461)

Keep in mind that high- and maximal-strain doesn’t just mean “high-intensity.” Strain accounts for both intensity and duration and thus, it’s possible that a longer exercise session at a moderate intensity could be more straining than a high-intensity session that lasts only 10 minutes. Important context for when we discuss the implications of the study later on.

Timing was calculated relative to each person’s habitual sleep onset (their average bedtime, adjusted for weekday vs. weekend across and across seasons). For example, if someone usually slept at 11:00 PM, a 7:00 PM workout would be considered 4 hours before sleep.

Exercise’s effects on sleep onset were timing and dose-dependent. Doing any exercise—but particularly high- and maximal-strain exercise 6 or more hours before habitual bedtime actually promoted an earlier sleep onset compared to not exercising or lower strain levels (somewhere in the range of 2–6 minutes on average).

When completed 4 hours before sleep, only light strain exercise improved sleep onset—other strain levels were essentially neutral in regard to sleep when performed in this time frame. However, within the 4-hour window before sleep, doing any type of exercise was associated with a later sleep onset in a strain-dependent manner. Moderate- to maximal strain exercise reduces sleep onset by ~8–20 minutes if you do it 4 hours before bed and 11–36 minutes if you do it 2 hours before bed. Exercising at a higher intensity at or after your regular bedtime (likely relevant for athletes engaging in late-night competition) was worst for sleep onset, delaying it by as much as 80 minutes when performed at this time.

Tl;dr: Exercise of any intensity and duration performed less than 4 hours before your normal bedtime means you’ll get to bed later than normal.

Similar findings were observed for sleep duration with a few exceptions. Exercise of all strain levels increased sleep duration as long as it was performed 6 or more hours before exercise. That means you’ll sleep longer on days you exercise compared to days that you don’t (and the higher the strain, the better—you gain about 7 extra minutes for a maximal-strain exercise).

Exercise within 4 hours of bedtime, however, and sleep duration suffers for all levels of exercise strain—with effects ranging from a 9-minute reduction with exercise performed 4 hours before bed, a 30-minute reduction with exercise 2 hours before bed, and an 80-minute reduction for exercise performed at one’s habitual bedtime. These effects are dose-dependent: the closer you exercise to bed and the higher the exercise strain, the shorter your sleep that night.

This next finding surprised me—sleep quality was significantly impaired at all levels of exercise intensity when exercise was performed within 8 hours of bedtime, other than when light exercise was performed 0–4 hours before habitual bedtime (when it had a slight quality-enhancing effect). Maximal strain exercise, even if it concluded within the “safe” 4–6 hour window before bed, led to a ~0.5% reduction in sleep quality. Similar to the other sleep outcomes, effects scaled with the intensity of exercise and proximity to bedtime—higher strain scores closer to bed were worse for sleep quality than lower strain scores (although all were detrimental).

Finally, let’s take a look at two measures of overnight autonomic arousal (heart rate and HRV).

Any exercise at any time of day led to a higher overnight heart rate and a lower (worse) HRV compared to a day without exercise. For example, exercising 6–10 hours before bed increased overnight heart rate by around 1–2 beats per minute (higher heart rate for higher strain). Exercising 2–4 hours before bed elevated heart rate by around 2–4 beats per minute (again, higher strain led to a higher heart rate). Completing a maximal strain exercise session at your habitual bedtime or (god forbid) 2 hours after could elevate overnight heart rate by as much as 7–14 beats per minute. HRV values could be as small as 1 unit lower if light exercise was performed 6–10 hours before bed to as high as a 10-unit reduction at higher strain levels in the 2–4-hour window before bedtime.

Tl;dr: On an exercise day, expect overnight heart rate to be higher and HRV to be lower—suggesting elevated sympathetic arousal.

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I just threw a lot of numbers and statistics your way, but for a good reason—they give some practical idea of how exercise of varying intensities will impact your sleep, helping you make an informed decision about how (and when) to workout. Most of what was observed won’t be too surprising to most of you—exercise too hard too close to bedtime and you’re probably not going to sleep as well. Exercise fairly hard early on in the day and you’ll likely sleep better. It’s worth noting that many of these effects were (while significant) quite minor—maybe you’re not at all concerned about a 5–10-minute reduction in sleep duration and exercise is worth the tradeoff.

What did surprise me was that any type of exercise worsened sleep quality no matter when it was performed…at least in comparison to a day without exercise.

This suggests that even for regularly active people, exercise is stressful and the body might take 24 hours or more to fully recover. This runs counter to the narrative that exercise acutely improves sleep quality—this study would suggest you sleep better on an off day (if you’re inclined enough to take one).

This leads me to a major strength of the study—the participants represented healthy and physically active adults (regular exercisers), meaning that even well-trained people can compromise their sleep by exercising hard close to bedtime. Fitness might not protect you, but being less fit might make these effects even more pronounced. If you’re in an early phase of training, be especially careful about exercising close to bedtime.

Now’s the time for some practical recommendations. If you’re someone who has to exercise after work (which might necessitate performing a workout in the 2–4-hour window before sleep), these results suggest keeping the strain low, which could mean performing a longer low-intensity workout or a shorter high(er) intensity one. If you’re someone who works out twice per day, make sure your hard workout is in the morning and leave your easy session for the PM.

What happens if you do perform high-strain exercise in the late afternoon or night? A few things. Try to avoid caffeine and other stimulants beforehand—this will compound the effects of exercise on sleep. It might also be a good idea to try to have your recovery meal soon after working out (and as far from bedtime as possible). Lastly, find some way to bring your core body temperature down, because that’s probably the biggest contributor to poor sleep quality and elevated overnight heart rate.

I really enjoyed this study because it illustrates that exercise can be a double-edged sword when it comes to sleep—enhancing it or deterring it depending on several different variables. It’s a friendly reminder (I need it as much as anyone) that statements like “exercise improves sleep” are inaccurate and misleading. As always, “it depends.”

Thanks for reading. See you next Friday.

~Brady~

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1. Leota, J., Presby, D.M., Le, F. et al. Dose-response relationship between evening exercise and sleep. Nat Commun 16, 3297 (2025). https://doi.org/10.1038/s41467-025-58271-x