Working Out This Many Hours Before Bedtime Will Negatively Affect Sleep Quality

Sometimes, the day gets away from you. Your window for exercise shrinks smaller and smaller until it's dark outside before you've had the chance to roll out your mat or lace up your sneakers.

While working out is a great way to support your health and get endorphins flowing at any time of day, we've long suspected that doing it too late can negatively affect sleep.

For their systematic review and meta-analysis, the team, led by Emmanuel Frimpong, Ph.D., sifted through existing research on how high-intensity exercise before bedtime affects nighttime sleep. After weeding out research that was biased or unreliable, they were left with 15 studies that included 194 participants. All participants were healthy adults (ages ranged from 18 to 50) who did not report sleep difficulties.

They determined that overall, exercising at night did not disrupt nighttime sleep in most people—with one caveat. "When exercise ended less than two hours before bedtime, sleep was negatively impacted," Frimpong said in a news release. "It took longer for participants to fall asleep and sleep duration decreased."

Exercise that was completed two to four hours before bed did not disrupt sleep in the same way. In fact, Frimpong explained, "Our analysis showed that when exercise ended two hours before bedtime, there were sleep benefits, including the promotion of sleep onset and increased sleep duration." Although, interestingly enough, those who exercised two to four hours before bed spent less time in the REM sleep stage—where most dreaming occurs—compared to a no-exercise control.

While it's not totally clear why working out at night can promote better sleep, it would make sense that doing so would tire you out for bed. There's also the possibility that the influx of endorphins that come during exercise could help put you in the right mindset for stress-free slumber.

However, anyone who's ever ticked off a tough HIIT class knows that high-intensity exercise dramatically raises heart rate and body temperature. And as psychologist and behavioral sleep medicine specialist Shelby Harris, PsyD, DBSM, previously told mbg, that sweaty, revved-up state is not conducive to relaxation. It's important to give yourself enough time—at least two hours, according to this review—to cool down and lower your heart rate before bed.