I came upon this reality in using the Whoop Strap, a buzzy tracker that measures the body's strain, sleep, and recovery in precise detail, I learned that when I hit the hay, a whole lot more happens internally than I ever realized. In this case, it's that slow wave sleep quality not only affects how I feel the next day, but also how I perform (on a fitness and a cognitive level). SWS is the stage when "the body produces 95 percent of its daily supply of growth hormones," Whoop's site reports. And, especially because I hadn't heard of this stage of sleep prior to using the tracker, learning that it's just as essential as its popular sleep-stage cousin REM particularly blew my mind.
Why slow wave sleep matters
"When we fall asleep, we enter and re-enter various stages, each marked by different brainwaves and different things physiologically that are happening," says Rebecca Robbins, PhD, sleep researcher and co-author of Sleep for Success, who notes that the distinction between REM and non-REM sleep is of note. "Each offers a different contribution to our waking success."
REM sleep is all about restoring your brain. SWS, which is non-REM, on the other hand, restores your body. "In general, slow wave sleep is the physically restorative part of sleep, and REM is mentally restorative," says Emily Capodilupo, director of analytics at Whoop. "Slow wave sleep monitoring for athletes is particularly important. If you have a really tough workout, you're breaking down your muscles. And we get stronger as we sleep because we repair that damage and make the muscles stronger than before." So if you're not getting enough of this specific sleep cycle, you're hindering how strong you're going to get, no matter how hard you're training.
If you're not getting enough of this specific sleep cycle, you're hindering how strong you're going to get, no matter how hard you're training.
Interestingly enough, the body itself acts as a scale to decide how much SWS versus REM it requires. "If you get more REM sleep after a mentally challenging day, it's normal, and you'll get more slow wave after one that's physically challenging," says Capodilupo.
Dr. Robbins echoes this, calling upon research about childhood development for more evidence. "This is why the majority of sleep that young children get is dominated by stage three, or slow wave sleep, which goes to show the importance of this stage from a muscular growth and regeneration standpoint." Beyond recovery, research also connects slow wave disruptions to several diseases, including Alzheimer's, and lowered memory function, further highlighting its importance.
How to maximize slow wave sleep
While your body ultimately decides the distribution of REM and non-REM sleep it gets, a strong sleep environment is important for making the most of both types. "We found in our data that the more consistent your sleep is, as far as your bed time and wake-up time, the more slow wave sleep you'll get," says Capodilupo.
Want examples of behaviors that don't facilitate a strong sleep-game situation? Drinking before bedtime. Because SWS happens earlier on in the cycle (REM is a later stage), if you drink close to bedtime, you're tasking your body with digesting the alcohol while you're sleeping, which can mean bad news for the quality of your zzz's. "When your body's processing alcohol, it's incompatible with slow wave sleep," says Capodilupo. "By the time it has left your system and has been fully processed, that's typically when you see REM sleep, so you get very little from the other, just-as-important stages," she says. Dr. Robbins adds that caffeine in the system can have a similar effect.
Even though SWS is an essential part of your night, you likely don't actually spend all that much time in that stage. "Spending 15 percent of the night in slow wave is a good number," says Capodilupo. "You'll typically see as the night goes on, the duration of each instance of slow wave gets shorter and shorter, and most happens in the beginning of the night."
Since I've been tracking my sleep like a hawk, I have noticed that a solid night's rest—complete healthy slow wave sleep stages—makes me feel ready to go in the a.m. and stay that way throughout the day. And, thanks to the power to track the time I spend sleeping each stage, combined with knowledge of what each stage supports physiologically speaking, I feel more empowered than ever to take my nighttime routine into my own hands and make expert-supported adjustments as necessary. Basically, grade-A physical and mental recovery, here I come, night after night.
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