Tell me if this sounds familiar: You go to bed intending to sleep at least seven hours so that you’re ready to start the next day bright and early. When your alarm goes off, you think, “Great, I got so much sleep,” only to find yourself yawning non-stop and fending off the urge to lie back down.
What gives?
Well, optimum sleep is more than just the number of hours you get each night. It’s also a bit of a complicated puzzle involving sleep cycles and stages, each with its own purposes and benefits. And if you’re trying to boost the quality of your shut-eye, looking at your deep sleep can be particularly beneficial to understand how it all fits together.
What Is Deep Sleep?
In short, deep sleep is an important stage in the sleep cycle, and it’s key for keeping your mind and body in tip-top shape. But what does that mean exactly?
While you might be ready to check out when your head hits the pillow, the work is just beginning for your body and brain. From the time you declare lights out to hitting snooze on your alarm, your body will likely go through four to five (and sometimes six) sleep cycles, with each cycle lasting about 90 to 120 minutes, according to the Cleveland Clinic.
Experts believe each sleep cycle has two phases: non-rapid eye movement (NREM), which has three stages, and rapid eye movement (REM), which has one stage, according to the National Institutes of Health (NIH).
When you go to sleep, you’ll typically start at NREM stage 1, then move between NREM stages 2 and 3, and then cap off the process with REM sleep before starting a new cycle, per the Cleveland Clinic. That said, not everyone will move through this cycle in chronological order, and some people may skip a stage or two, according to the Mayo Clinic. That’s what makes sleep so individual.
Deep sleep (NREM stage 3), which is what we’re focused on here, is also called slow-wave sleep because of the characteristically slower delta brain waves. You’ll typically spend more time in this restorative sleep stage earlier in the night, per the NIH.
During deep sleep, your heart and respiratory rates are slow and your muscles are relaxed, allowing you to benefit from restorative snoozing that’s necessary for overall health (more on that below). This state of deep relaxation is also why it’s difficult to wake someone up during deep sleep.
Why Is Deep Sleep Important?
So, what exactly is so important about deep sleep? During NREM 3, the body undergoes significant physical repair and restoration, including muscle recovery, tissue growth, and the release of growth hormones. It’s also the stage where your brain processes and consolidates information and experiences from the day, helping you retain and make sense of new things.
What’s more, getting sufficient deep sleep can help your body fend off infections and illnesses, replenish energy reserves, and help you wake up feeling refreshed, energized, and ready to take on the day.
What Happens If You Don’t Get Enough Deep Sleep?
“The unintended consequences of sleep deprivation can be devastating,” says J. Rico Blanco, MSHA, Certified Director of Sleep Medicine for the American Osteopathic Association. “Physically, [those who are] sleep-deprived experience fatigue, low energy, and lack of alertness,” he explains.
When it comes to your mental state, Blanco says a lack of sleep can affect your overall mood, which can lead to being easily agitated, upset, or depressed. “Both contributing factors negatively affect your overall health, and when you combine lack of energy and depression, it equals unsatisfactory health outcomes,” he adds.
“Sleep deprivation, even one night, is known to have negative effects on cognitive function and physical function and performance,” explains Vernon Williams, MD, board-certified neurologist, sports neurologist, and founding director of the Center for Sports Neurology and Pain Medicine at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles. “There are also significant effects on mood and impulse control with resulting disinhibition,” he adds.
Even if you get the recommended total hours of sleep each night, too little time spent in the deep sleep stage can leave you feeling groggy, drained, and tired. That’s why your body needs slow-wave sleep the most. Unfortunately, it’s also the stage we naturally spend less time in as we age, which explains why so many of us describe ourselves as “light sleepers.”
Over time, Dr. Williams says insufficient deep sleep is felt to have negative effects on the glymphatic system, which is a kind of maintenance system that helps clear your brain of proteins and toxins such as tau and amyloid (which are implicated in some neurodegenerative conditions). “Poor or disordered sleep during the middle-age years has been associated with increased risk of dementia as you age,” he adds.
How Many Hours of Deep Sleep Do You Need?
While getting seven to nine hours of sleep per night is a standard recommendation for adults, it doesn't guarantee that you'll be getting a specific amount of deep sleep. Plus, there’s no one-size-fits-all or magic number of sleep hours that works for everybody.
To get a good idea of how many hours of deep sleep you need, we first need to look at the percentage of time you spend in each sleep stage. NREM stage 1 is short and light. In fact, you can expect to spend only a few minutes here, or about 5 percent of your total sleep time, before moving into stage 2, according to the Cleveland Clinic. While you’ll still be in the light sleep phase of the night, stage 2 is deeper than stage 1, and accounts for about 45 percent of your sleep hours. Once you cycle through stage 2, it’s off to the good stuff.
Deep sleep, which, again, is considered NREM stage 3, makes up about 25 percent of the total sleep time for adults. In other words, if you average seven to eight hours of shut-eye a night, your deep sleep should translate to about one to two hours of this time.
Finally, REM sleep makes up about 25 percent of your total time, similar to NREM stage 3. It’s also where you’ll spend most of your time dreaming.
Keep in mind that these are rough guidelines, and individual sleep needs can vary. Some people may naturally have more or less deep sleep, and factors like genetics, lifestyle, and sleep disorders can influence it, too.
How Can You Tell How Much Deep Sleep You’re Getting?
So, how do you know if your time spent in bed is comparable to the guidelines? It’s hard to nail down specifics, but wearable smartwatches, bracelets, or headbands can give you a rough estimation by collecting and analyzing your snooze stats.
That said, to get exact data about your Zzzs, you need to do a medical sleep study, according to Johns Hopkins Medicine. These studies monitor brain waves to analyze your sleep throughout the night.
The Connection Between Deep Sleep and Exercise
Deep sleep helps us feel rested and ready for the day. It’s also when your body repairs and rebuilds tissues, strengthens the immune system, and consolidates memories. That’s why maximizing our time in this stage is so important.
One way to help increase the odds that you’ll increase and improve deep sleep is with exercise. Experts from Johns Hopkins Medicine suggest that moderate aerobic activity increases the amount of slow-wave sleep you get, with some people noticing a difference that same night after engaging in 30 minutes of exercise.
Blanco says adopting a new exercise program increases the likelihood of getting a better night’s sleep. That’s because getting active increases your ability to sleep more soundly, as our bodies require more sleep to restore and build muscle. Exercise also enhances self-efficacy, which, Blanco says, may increase overall life satisfaction and remove anxiety that can plague you when you’re trying to nod off.
Exercise also appears to improve the glymphatic system functions mainly during sleep, “which has significant potential ramifications for chronic neurological disorders such as Alzheimer’s, dementia, and Parkinson’s Disease,” Dr. Williams explains. What’s more, improvements to this system may also optimize general neurologic function throughout your lifetime. (Translation: Snoozing can help keep your brain healthy.)
And while exercise is known to improve the speed at which you fall asleep and improve sleep quality, Dr. Williams says it’s important to note that exercise too close to bedtime can impair sleep onset. (Just avoid vigorous exercise one hour before you hit the hay.)
Benefits of Deep Sleep for Physical Recovery
“Deep sleep is a necessary component of a healthy, balanced lifestyle,” Blanco says. Whether you’re a little active or very active, Blanco says everyone can benefit from a good night’s sleep. “Sleep should be restorative, allowing your body to repair muscle cells and replenish your overall health,” he adds. As mentioned above, deep sleep is crucial for recovering from a workout and getting your mind and body ready for the next one.
What’s more, sleep isn’t simply a “brain rest” state, Dr. Williams explains. “There are active physiologic processes taking place in the body that require sleep, particularly its relationship to circadian rhythms,” he says. “These processes affect nearly every cell, organ, and system in the body.”
The Role of Nutrition and Lifestyle in Deep Sleep
Eating too close to bedtime can be a tricky endeavor when it comes to your Zzzs. Choose certain foods, and you’ll be up tossing and turning with heartburn or indigestion. Sip on a caffeinated beverage late in the afternoon, and you risk staring at the ceiling counting sheep. But when it comes to deep sleep specifically, why does nutrition play such a critical role in how much we get?
Well, according to Dr. Williams, eating too close to bedtime can negatively affect deep sleep, as can caffeine and alcohol intake. Another area of concern, he says, are extreme diets that place you at risk for nutrient deficits and dehydration, which can negatively impact slow-wave sleep.
The good news is that healthy nutrition and lifestyle habits can optimize slow-wave sleep. “Eating a balanced diet, maintaining good hydration, regulating the time of food consumption, and avoiding heavy meals for a few hours prior to sleep onset is a good place to start,” Dr. Williams says.
Blanco agrees that dietary choices and lifestyle factors can positively or negatively affect deep sleep. “As a caffeine enthusiast, I have found that limiting my coffee intake to one to two cups daily before 1:00 PM allows me to fall asleep quickly and sleep more soundly,” he says. “If I stray from this routine and consume caffeine later in the day, my sleep becomes disruptive, and the effects can linger for a day or two.”
Blanco also says eating meals that are overly processed, as well as foods high in sugar or saturated fat, has an impact. One way to get a better night’s sleep, Blanco says, is to adopt a balanced diet. For example, he recommends the U.S. Department of Agriculture's (USDA) MyPlate five food groups, which includes fruits, vegetables, grains, protein, and dairy. “A solid foundation of foods [that are] high in fiber, nutrient-dense, and fresh can help your body restore and replenish resources used during the day,” Blanco says.
Beyond food and nutrition, Blanco really stresses that what we do daily directly affects our sleep. “I encourage people to live a life they are passionate about because that feeds the mind and body and can usher in a better night's sleep.”
The Takeaway
While getting seven to nine hours of sleep is a good general guideline for most adults, the number of sleep cycles you experience and the time spent in each stage, including deep sleep, can vary widely. That’s why focusing on overall sleep quality and making sure you get enough total sleep to feel rested and alert during the day is key. If you find you’re more tired or fatigued than normal, or you’re experiencing sleep disturbances despite getting what you believe to be an adequate amount of sleep, it may be worth consulting a healthcare professional or sleep specialist to evaluate your snooze patterns and address any potential sleep disorders.