We’ve said it before and we’ll say it again: Sleep is paramount in the quest for well-being. While working out might help you live longer, cumulative poor-quality sleep (for example, if you have sleep apnea and you’re peeing throughout the night) can have a seriously detrimental effect on your mental and physical state. That said, do you know what the cornerstone of poor-quality sleep is? It’s not a lack of dreams. Rather, the overall quality of your sleep is largely determined by how much deep sleep you get. To find out how much deep sleep you need, we chatted with a few of the industry’s leading sleep experts to take a dive into the subject. Ahead, learn what deep sleep is, how it benefits your health, and why it should be your number one priority moving forward.

What is deep sleep?

In order to understand deep sleep, you must first acknowledge the phases of the sleep cycle, as well as the stages of sleep. According to Sleepless in NOLA sleep consultant Nilong Vyas, MD, medical review expert at Sleep Foundation, the sleep cycle consists of two phases: REM (rapid eye movement) and NREM (non-rapid eye movement). “The NREM phase is further divided into three stages of sleep,” she explains. The three sleep stages are N1 (light sleep), N2 (medium sleep), and N3 (deep sleep). “The human body cycles through four to five stages of sleep every 90 minutes overnight,” Dr. Vyas says. “N1 is the lightest sleep, N2 is deeper and plays the most significant role in memory consolidation, and N3 is the deepest of the NREM phase.”

Why is deep sleep so important?

While every stage of sleep plays a role in overall health and wellness, deep sleep is the most important because it’s responsible for strengthening the immune system, repairing tissue, and releasing growth hormone. All in all, it’s when the body heals itself, Dr. Vyas says.

So perhaps you’ve been feeling under the weather and can’t seem to muster up any energy—deep sleep will help; maybe you had a super long day that started with a killer workout and felt absolutely wiped by the end of it—deep sleep will help; perhaps you’re navigating a breakup or other stressful life event—deep sleep will help.

But here’s the thing: It doesn’t only have an immediate impact on a person’s well-being—it’s cumulative, meaning that it can offer benefits that last a lifetime, or at the very least, extend your lifetime.

“During deep sleep, the brain’s ‘waste management system’ comes in and removes certain proteins that if not removed are thought to lead to Alzheimer’s and cognitive decline,” says sleep expert and clinical psychologist Michael Breus, PhD, aka The Sleep Doctor.

Which is better: REM or deep sleep?

While REM sleep is a different phase of the sleep cycle, it’s considered the fourth and final stage of it. Since rapid eye movement is tied to dreaming, many people mistakenly assume that it must be a very deep form of sleep in which the subconscious has the uninterrupted ability to soar. In reality, Dr. Vyas says that REM sleep is actually considered to be less restful.

That doesn’t mean that REM sleep isn’t important, though. “REM and deep sleep are completely different and both necessary for different reasons,” Dr. Breus says. “As an example, during REM sleep is when you move information from your short term memory to your long term memory, and when you process emotions; deep sleep is needed for more physical areas.” Though, it plays a role in long term memory, too.

How much deep sleep do you need each night?

TL;DR: It depends. How much sleep you need depends on your age, gender, medical condition, fitness level, and environment, Dr. Breus says. “As a general guideline, we recommend seven to nine hours of good quality sleep, but this can vary widely,” he adds. (Not sure where you stand? Check out our story on sleep calculators.)

If you’re over the age of 65, Abhinav Singh, MD, FAASM, medical director of the Indiana Sleep Center and a medical review expert at Sleep Foundation, says that seven to eight hours will typically suffice.

No matter your age, if we’re breaking it down by stage, Dr. Singh says that 20 to 25 percent of your total sleep should be deep sleep, while another 20 to 25 percent should be REM.

What happens if you don’t get enough deep sleep?

Considering the vital role it plays in our overall well-being, Dr. Breus reveals that not getting enough deep sleep is what’s officially considered sleep deprivation. “Sleep deprivation affects every organ system and every disease state—literally everything you do, you do better with a good night’s sleep,” he says.

More specifically, Dr. Breus points out that a lack of deep sleep can lend to a notable physical and cognitive decline, including slower reaction time, lower testosterone, memory issues, riskier decisions, trouble focusing, and more. And then there’s the emotional impact of sleep deprivation. When you don’t get enough of it, you’ll become more anxious and could experience deeper depression. Together, all of these side effects make one thing very clear: Deep sleep should be your priority—each and every night.

When does deep sleep take place?

While a full sleep cycle is roughly 90 minutes long and is comprised of all four sleep stages (N1, N2, deep sleep, and REM sleep), the amount of time you spend in each stage changes throughout the night. According to The Sleep Foundation, you typically get the most deep sleep during the first half of the night. Then, by the later sleep cycles, you spend less time in the N1, N2, and N3 stages, and more time in REM sleep.

What happens if you wake up during deep sleep?

Since the N3 stage is the deepest sleep stage, it makes sense that it’s the hardest to snap out of. According to Dr. Breus, the deep sleep stage is the hardest to awaken from. If someone does wake up during this stage, he says it’s not uncommon to feel especially groggy. “This experience is what’s known as sleep inertia,” he reveals on his website. He compares it to Isaac Newton’s Law of Inertia and points out that unless a pressing force is at play to entice someone to wake up, it’s totally normal to wish you were still asleep—or even fall back asleep in the process. This is a sign that you’re within a deep sleep stage and need it to feel rested.

If you continually wake up each morning wishing you could just sleep a little longer, however, it might be time to re-evaluate your sleep hygiene and routine, as certain habits can make falling and staying asleep much easier. For example, eating a snack comprised of foods that help sleep, taking OTC sleep aids (like the Olly Sleep Gummies, $13), wearing breathable cotton PJs (like the Printfresh Bagheera Sleep Shirt, $118), sleeping on soft, moisture-wicking sheets (like the Purple SoftStretch Sheet Set, $189), and waking up with a sunrise alarm clock (like the Hatch Restore 2, $200) can all make a big difference in the quality of your sleep.

If after adjusting your sleep routine you still find that falling and staying asleep is a challenge, or that you’re experiencing poor sleep quality overall, you may want to consult a doctor to discuss cognitive behavioral therapy for insomnia, tests for sleep disorders, or the possibility of prescription sleeping pills and best next steps overall.

—reviewed by Angela Holliday-Bell, MD