“You might have a great exercise routine and eat well all the time, but if you’re not getting enough sleep, it cancels out all your healthy behavior,” says James Maas, Ph.D., a sleep expert and author of Sleep to Win. “Sleep is the most important thing we can do to reset our brain and body,” he says.
But turning in at night doesn’t mean you’re turning off—sleep is actually one of your body’s most active states. “A lot of really important things happen during sleep,” says Jennifer Martin, Ph.D., clinical sleep psychologist and associate professor of medicine at UCLA. “Sometimes I hear athletes talk about sleep as a form of passive recovery when actually I think about it as active recovery.”
There are four stages of sleep: non-rapid eye movement stage one, two, and three, and finally REM sleep. “Stage three is when our brain waves are slow and our body is active, and REM sleep is when our brain is active,” says Martin. Matt Berenc, director of education at the Equinox Fitness Training Institute, adds: “Non-REM sleep is linked to physiological recovery like muscle repair and REM sleep is key in memory consolidation and cognitive ability and all the skills you need on a mental level.”
Here’s how busy your body is when it’s unconscious:
1. Your muscles recover from tough workouts….
When your body enters non-REM deep sleep, it signals your pituitary gland to secrete growth hormones that stimulate tissue growth and muscle repair. At the same time, circulation increases so there’s a boost in blood supply to your muscles, sending extra oxygen and nutrients to support growth and recovery. Says Berenc, “Sleep is essentially the cornerstone of the muscle recovery process.”
2. ...and learn how to do new things.
“During sleep, the brain moves short-term muscle memory—like a new dance step or a tennis serve—into long-term muscle memory, where you can more easily retrieve it later,” says Maas. “So the old saying ‘practice makes perfect’ only works if you get adequate rest afterward, meaning we should really be saying ‘practice with sleep makes perfect.’”
3. Your immune system is strengthened.
“While you sleep, your immune system is bolstered. So, when you later encounter bacteria, being well-rested makes you more likely to fend it off,” says Martin. In fact, a study found that people who slept fewer than six hours per night were four and a half times as likely to catch a cold when exposed to a virus compared to their counterparts who logged at least seven hours.
4. Cravings are controlled.
Research found that people who didn’t get adequate sleep showed decreased levels of leptin (which manages feelings of satiety) and increased levels of ghrelin (which stimulates appetite). “When people are sleep deprived, they crave more high-fat, high-sugar, calorie-dense foods so that can make it really difficult to stick to a good nutrition plan, despite your best intentions,” says Martin.
5. You'll get a healthy glow.
Your complexion can often give away whether you’ve had a fitful night (pale and sallow) or a solid eight hours (rosy and glowing). When you sleep, “blood flow and oxygen to the skin increases, aiding skin repair processes responsible for skin elasticity and youthful appearance,” says Jeffrey Benabio, M.D., a dermatologist in San Diego, California. “It also helps repair some of the damage that has been done by UV light during the day. Sleep is a natural anti-inflammatory that can help with skin conditions such as acne, psoriasis, and eczema.”
6. Toxins are flushed out.
“ Research shows that during sleep our brain clears out metabolic waste including a toxic protein called beta-amyloid. This is important because beta-amyloids are one of the markers of Alzheimer’s,” says Martin. Sleep also strengthens the neurons that keep your memory sharp: A study found that skipping out on shut-eye can slow brain activity and stunt neural connectivity in the hippocampus.
7. Your heart beat slows and blood pressure drops.
As you slip into stage three non-REM sleep, your blood pressure naturally drops about five to seven points, the way it does when we meditate, says Maas. And a study indicated that consistently getting adequate sleep can reduce blood pressure over time and prevent hypertension. Adds Maas, “One extra hour of sleep per night decreases the risk of artery calcification—a leading cause of heart disease—by 33 percent.”