The Beginner’s Guide to Slow-Wave Sleep

Medically Reviewed
Key Takeaways

Slow wave (N3) sleep is the most restorative stage of sleep, and most important for waking up feeling refreshed.

Deep sleep is critical for our body to repair itself, form new memories, reduce anxiety, and maintain a healthy immune system.

To increase the amount of N3 sleep you're getting, try taking a hot bath or shower before bed, keeping your sleeping environment cool, listening to pink noise, and engaging in moderate aerobic exercise.

Medically Reviewed By
Dr. Anshul Gupta
Written By
WellTheory Team

Prior to the early 1920s, scientists believed sleep was passive, a state of low activity when a person's brain and body shut down for the night to rest and recover. Our understanding of sleep hadn't evolved much beyond the beliefs of the ancient Greeks, who viewed Hypnos, the god of sleep, as the brother of Thanatos, the god of death.

The scientific study of sleep had a breakthrough year in 1953 when Eugene Aserinsky and Nathaniel Kleitman of the University of Chicago observed eye movements during sleep and discovered two distinct forms of sleep: rapid eye movement (REM) sleep and non-REM (NREM) sleep. Through their research, the duo found that REM sleep was correlated with dreaming and increased brain activity (interestingly, closer to that of someone awake rather than unconscious).

By contrast, NREM sleep was characterized by deep, slow-wave activity and eventually broken into three distinct stages itself: N1, N2, and N3. As we fall asleep and progress through the sleep stages, our brain waves progressively slow and increase in amplitude.

We now know that our bodies alternate between both REM and NREM sleep as part of the sleep cycle and that the difference between those states is as profound as the difference between periods of sleep and wakefulness.

Today, we're uncovering more about the deepest stage of sleep (stage N3), also known as the reason why you might experience a “sleep hangover” if you oversleep your afternoon nap alarm.

What Is Slow-Wave Sleep and Why Should I Care?

NREM sleep makes up the majority of our total sleep time at night

Although both REM and NREM are important for different functions in our bodies, NREM sleep typically comprises 75% to 80% of total sleep each night. An average 8-hour night of sleep contains around five of these NREM-REM sleep cycles, each lasting ~90 minutes. Within that, slow-wave sleep (SWS) occurs mostly in the first third of the night and constitutes 10% to 20% of total sleep duration. (Source)

Slow-wave sleep is when your body repairs and renews itself

To understand why N3 is important, think of your body as a factory that performs several vital functions. As you drift off to sleep, your body begins its night-shift work — performing maintenance and repair all the way down to the cellular level. At the beginning of the N3 stage, the pituitary gland releases human growth hormone (HGH), which stimulates tissue growth and muscle repair. During this regenerative stage, the body repairs and regenerates tissue, builds bone and muscle, strengthens the immune system, and flushes out built-up waste products. (Source, Source, Source)

Slow-wave sleep is the most restorative sleep stage

N3 is considered to be the most restorative stage of sleep, and most important for waking up feeling refreshed. This may be why the majority of N3 sleep occurs during the first half of the sleep period — if you don’t get the full amount of sleep you need, you’ll still get a good dose of N3. During N3 sleep brain activity declines to 75% below normal wakefulness level, which explains why you typically feel groggy and disoriented if you wake up during deep sleep (fun fact: this phenomenon is scientifically known as “sleep inertia”). (Source, Source)

Slow-wave sleep declines with age

The older you get, the less NREM sleep you'll have. People under age 30 typically have about two hours of restorative sleep every night, whereas older adults over 65 might get only 30 minutes. N3 sleep continually decreases with age to the point that some elderly people may have no measured N3 sleep at night at all. (Source, Source)

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What Does the Research Show About Slow-Wave Sleep?

Slow-wave sleep acts as a natural anti-anxiety agent

UC Berkeley researchers showed that people who received more slow-wave NREM sleep had lower anxiety levels compared to those who had a sleepless night. Their study showcased the importance of NREM sleep specifically and its role in calming an overanxious brain. nterestingly, studies have shown that increased cortisol levels are correlated with longer periods of slow-wave sleep and shorter periods of REM sleep. (Source, Source)

Slow-wave sleep plays a key role in immune function

Studies have shown that when fighting off an illness, your body may increase the amount of time spent in NREM sleep while deprioritizing REM. This occurs for two key reasons:

  • Our body needs more frequent repairs to recover when “at war”
  • The reduced energy demand of NREM sleep leaves more fuel for maintaining a fever — an energy-intensive process. (Source)

Slow-wave sleep is important for memory consolidation

Deep sleep, often referred to as “sleep-dependent memory processing,” is critical for forming and encoding new memories in our brain. Slow-wave sleep improves declarative memory specifically, which includes common knowledge and information about everyday events. It's not surprising then that aging and its associated decrease in deep sleep is accompanied by declines in cognitive function and substantive memory loss, especially in age-related disorders such as Alzheimer's disease. (Source)

N3 sleep is compensatory

Sleep deprivation studies have consistently demonstrated that your body will compensate for slow-wave sleep if it doesn't get enough. So on an average night, if you typically have about 100 minutes of slow-wave sleep, but then miss a night of sleep, your body will compensate by having around 200 minutes of slow-wave sleep on your recovery night. (Source)

So What Should I Do About Slow-Wave Sleep?

Take a bath before bed

Those of you who know me well probably aren't too surprised to hear this one, since I'm a huge proponent of a nightly bath. A hot shower or sauna will work too. Heating the body one to eight hours before bed increases slow-wave sleep and NREM consolidation ― a phenomenon known as the “Warm Bath Effect.” (Source)

Keep your cool

After heating your body up, you'll want to cool off and sleep in an ambient environment (between 60 and 67 degrees). Typically, our body temperature decreases by one or two degrees as part of the sleep initiation process, enabling us to conserve energy and direct it to other parts of the body. (Source)

Try a low-carb diet for more deep sleep

A study published in the medical journal The Lancet showed that consuming a high-carb, low-fat diet led to less slow-wave sleep compared to a low-carb, high-fat diet or a normal balanced diet. Whatever your diet, make sure you are getting plenty of magnesium, which has been shown to promote relaxation and sleep by helping calm the central nervous system. (Source)

Pink is the new white (noise)

Pink noise, which is deeper and has slower waves than white noise, enhances brain activity that’s associated with deep phases of sleep, known as delta waves. A study by Northwestern Medicine found that pink noise boosted deep sleep in patients with mild cognitive impairment. (Source)

Sweat can help you sleep

Studies have shown that moderate aerobic exercise increases the amount of slow-wave sleep you get. Our bodies appear to prioritize deep sleep the night after an intensive workout — in the same study, individuals with greater self-perceived exertion during exercise had decreased light sleep and increased deep sleep compared with those who reported less self-perceived exertion. (Source)

Where Can I Go to Learn More About Slow-Wave Sleep?

On the connection between sleep and Alzheimer's

In this podcast episode, Matthew Walker, professor of neuroscience at UC Berkeley and author of “Why We Sleep,” talks about sleep in conversation with physician Peter Attia — discussing the different and important roles of REM vs. non-REM sleep, how memory consolidation works, and the dangers of chronic sleep deprivation.

For DIY deep sleep tracking

If you're curious to see how much N3 sleep you're getting each night, check out this NYTimes Wirecutter article featuring sleep-tracking apps. Spoiler alert: their vote is for SleepScore. (Source)

For binaural beats

When a slightly different frequency of sound is played into each ear, the brain perceives a fixed beat or frequency, known as a binaural beat. Binaural beats in the delta waves range (1 to 4 Hz) are thought to be associated with deep sleep and relaxation, although a recent study suggests this may be largely a placebo effect. Relax Melodies is one app that lets you create your own soothing sleep mix with these beats, white noise, nature sounds, and others. (Source)

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