What happens to your muscles when you snooze, according to sleep experts


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Over the past three years, the recovery industry has been booming. Between stretch studios, $300 percussive therapy devices, and wearables that track your every move, it’s easy to drop hundreds of dollars to get your feet back under you after a hard workout. While all of this tech is undoubtedly flashy and effective, in a newsflash to exactly no one, the best way to recover is through sleep.

In order to get stronger, your muscle fibers have to tear on a microscopic level and when they grow back, you gain strength. This process primarily happens while you’re sleeping, so when you don’t catch enough zzzs, it isn’t able to happen properly—studies show that both mental and physical performance are impaired after a single night of sleep deprivation. “Without sleep, muscles become more inflamed and begin to break down their own protein to provide building blocks—aka nitrogen and other protein components—to the rest of the body, which needs more protein to function during sleep deprivation,” says Ben Smarr, science advisor to Oura and assistant professor at UCSD Bioengineering & Data Science. “Sleep allows healing, but deprivation causes breakdown, making getting good sleep doubly important.”

Not only that, but when you deprive your body of its most regenerative process or if you get poor sleep, you’re taking away a key element of recovery. “Non-REM sleep is associated with the highest levels of growth hormone release during a day, allowing muscles to heal and grow,” says Smarr. Research has shown that your body produces 95 percent of its Human Growth Hormone for the whole day during slow wave sleep, which is the cornerstone of recovery. Remember those muscle tears? While you’re asleep, HGH helps repair them by way protein synthesis (aka generating new muscle cells). “If you don’t give yourself that opportunity to get into solid sleep, then you don’t repair those muscles,” says Emily Capodilupo, the director of analytics at WHOOP. If the muscles aren’t able to repair themselves, she explains that the micro tears can compound on each other, and turn into overuse injuries, which obviously no one wants.

So what’s actually going on with your body as you snooze? “During sleep, the muscles are going through a regenerative process,” says Jeff Monaco, Gold’s Gym director of education. This is happening on a few different levels, most of which occur when you reach slow-wave sleep. Everyone’s muscles repair during sleep, but the way they repair depends on how you’re working out. “If an individual is engaged in heavy resistance training, the muscles fibers will suffer acute damage, and the body will response by rebuilding those muscle fibers stronger and larger,” says Monaco. “If an individual is engaged in endurance training, the body will respond by increasing the oxidative capacity of those muscle fibers through increases in mitochondrial density and size.”

In addition to aiding in necessary muscle repair after a workout, sleep is also important for keeping your endocrine, immune, and nervous systems in check, each of which are important to keeping your body working properly. “If these systems are functioning at full strength that means your muscles cannot function at full strength either,” says Monaco. He points to a study. that shows that fatigue in the central and peripheral nervous systems can result in muscle fatigue, even if you haven’t actually fatigued that muscle through a workout or some other type of exertion.

If you’re looking to keep track of these things, that’s where wearables come in—which are admittedly slightly more expensive than the $0 that a good night’s rest will cost you on its own. Devices like the WHOOP ($30/month) and the Oura Ring (from $299) are able to monitor how many hours of sleep you get as well as the quality of that sleep, and use that data to give you information about your performance potential. They’ll also tell you when it’s a good idea to skip your morning workout in exchange for a few extra hours of shuteye. “Using a tracker lets you check in with your body each morning and think about what you did the day before that might have contributed to your sleep quality,” says Smarr. “Making more informed choices can have a huge impact on sleep, just like it can on diet and exercise.”

On the WHOOP and Oura platforms, making those good choices might also mean skipping a workout here or there. The devices take sleep, activity, and recovery into account, and deliver a readiness score a based on the results. This number lets you know how “ready” you are to perform, and if it’s below a certain threshold it’s a good indicator that you should take it easy. Sleep is vital to every major system in your body, and getting enough of it is important to keep them all functioning properly individually and together…. in and outside of the gym. And if that means snoozing your alarm for an extra hour for the sake of your “recovery”? So be it.

If you’re struggling to get to sleep, this one thing will help it happen faster. And here’s how you can tell if you’re tired, or dealing with legitimate fatigue.

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