The Deep Clean of Deep Sleep

Medically Reviewed by Sabrina Felson, MD on May 07, 2022

It takes a lot of energy to stay awake. Good quality deep sleep strengthens your memories and your immune system. And research shows it may help clear out toxins from your brain.

Experts don’t fully understand the need for sleep. But they know that a lack of it raises your chances of mental and physical health problems. That includes depression, heart disease, and diabetes. It’s why you shouldn’t skimp on a good night’s rest.

What Is Deep Sleep?

You can divide sleep into two types: rapid eye movement (REM) and non-rapid eye movement (NREM). You cycle in and out of NREM and REM every 90 minutes or so. This happens about five times a night.

Your deep sleep happens in stage three of NREM. That’s often called N3 or slow-wave sleep (SWS). You’ll get most of your SWS in the first half of the night. It’ll be really hard for someone to wake you up. But if they do, you’ll feel a little confused. That’s called sleep inertia.

During deep sleep:

  • Your heartbeat and breathing slow down
  • Your blood pressure drops
  • Your muscles relax
  • You respond less to external stimuli, like loud noises
  • Your brain waves change

What Are Slow Brain Waves?

When you’re awake, your brain waves move fast and are all over the place. That’s because you’re responding to a lot of different activity. But during deep sleep, you mostly tune out the outside world. Your brain cells, called neurons, fire less often. You get slower, larger brain waves called delta waves. During this slow-wave activity (SWA), your brain waves are more predictable.

Experts think SWA helps wash out waste from your brain. That’s because more cerebrospinal fluid flows in and out. It’s the liquid around your brain and spinal cord. But more research is needed to know if deep sleep causes this process.

How Long Does Deep Sleep Last?

You’ll spend less time in N3 as the night goes on. But about 20% of your overall sleep will be deep. Certain things affect how much you’ll get. That includes your age. If you’re under 30, you’ll get about 2 hours of deep sleep a night. People over 65 may only get 30 minutes.

Deep Sleep and Learning

When you take in new information, you store it for a short time. That’s called encoding. Deep sleep helps make those memories stronger. That’ll help you recall info later. 

Experts explain this with a theory called active system consolidation. Here’s how it works:

While you sleep, your brain replays what you learned earlier. Certain brain rhythms work together to transfer that new knowledge from a part of your brain called the hippocampus to a different area called the neocortex. Along with your slow delta waves, you get fast bursts of brain activity called sharp-wave ripples and sleep spindles. Research suggests this neural “crosstalk” is how sleep helps memories go from short-term to long-term storage.

Deep Sleep and Memories

SWS also helps reorganize your memories. Experts explain this housecleaning method with a theory called synaptic homeostasis. Scientists think the brain frees up space and energy while you sleep. The idea is that you get rid of unneeded synapses. Those are connections formed when your brain cells talk to each other.

Experts aren’t exactly sure how your brain decides what to keep. But studies show sleep tends to favor memories that’ll be important in the future. 

On the flip side, a lack of deep sleep is linked to memory problems, especially in older adults. People who don’t get much SWS have higher levels of tau and beta-amyloid. Those are proteins in your brain linked to dementia and Alzheimer’s disease. Experts think deep sleep helps clear them out. But researchers are still studying how sleep and Alzheimer’s affect each other. 

Other Benefits of Deep Sleep

Sleep affects your whole body. Deep sleep is important for:

  • A healthy immune system
  • Heart health
  • Tissue and muscle repair
  • Hormonal balance
  • Metabolism

Are You Getting Enough Deep Sleep?

Some apps and activity trackers claim to measure your sleep quality. But they aren’t a good way to know if you’re getting enough SWS. Instead, ask yourself if you feel rested when you wake up. Your brain and body will usually let you know if need more deep sleep.

If you’re having trouble sleeping, talk to your doctor. They can help you find out what’s going on. But you may sleep better if you exercise regularly or try meditation. Even a warm bath before bed can help you snooze.

Experts are also studying new techniques to enhance deep sleep. They’re hoping to find ways to help people with memory problems.

One example is sound therapy. Some small studies found SWS improved in people who listened to “pink” noise.  That’s a sound described as flat or even. Researchers studied short, timed bursts of pink noise. That isn’t something you can do at home. But there’s no harm in trying a pink or white noise app.

Show Sources


Frontiers in Neurology: “Functional Anatomy of Non-REM Sleep.”

Science: “Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep.”

National Institute of Neurological Disorders and Stroke: “Brain Basics: Understanding Sleep.”

MEDtube Science: “The Neuroprotective Aspects of Sleep.”

International Journal of Endocrinology: “Sleep and Metabolism: An Overview.”

Roneil Malkani, MD, director, Sleep Medicine Foundation; assistant professor of neurology, Northwestern University.

Physiological Reviews: “The Sleep-Immune Crosstalk in Health and Disease.”

StatPearls: “Physiology, Sleep Stages.”

National Sleep Foundation: “How Sleep Adds Muscle,” “What Happens When You Sleep?”

Cleveland Clinic: “Sleep Basics,” “Why “Pink Noise’ Might Just Help You Get a Better Night’s Sleep.” 

Frontiers in Psychology: “Sleep smart -- optimizing sleep for declarative learning and memory.”

Nature Communications: “Bidirectional prefrontal-hippocampal dynamics organize information transfer during sleep in humans,” “Deep sleep maintains learning efficiency of the human brain.”

Nature Neuroscience: “Prefrontal atrophy, disrupted NREM slow waves, and impaired hippocampal-dependent memory in aging.”

Science Translational Medicine: “Reduced non-rapid eye movement sleep is associated with tau pathology in early Alzheimer’s disease.”

Trends in Neuroscience: “Slow-wave activity enhancement to improve cognition.”

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