Deep Sleep

Last Updated: September 20, 2023

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Deep sleep, the third stage of the sleep cycle, is necessary for repairing and restoring bodily functions. Getting enough deep sleep is crucial to help with memory, musculature repair, growth, cell regeneration and immune function. A lack of deep sleep can negatively impact memory, learning, and many major health concerns, such as heart disease and Alzheimer’s.

What is Deep Sleep or Slow Wave Sleep?

Deep sleep is also known as slow-wave sleep or delta sleep. Deep sleep occurs in the third stage of sleep: non-rapid eye movement (NREM) sleep.

During deep sleep, brain waves are the slowest, with the electrical activity in the brain appearing in long, slow waves referred to as delta waves. For these waves to be considered deep sleep, their frequency must be 0.5 to 2 Hertz and make up a window of six seconds in a thirty-second window.

Deep sleep typically happens within the first hour of falling asleep and continues in shorter periods as sleep progresses. Bodily functions such as slow brain activity, breathing and heart rate slow down, and musculature relaxes during deep sleep.

Deep sleep is necessary to feel refreshed in the morning. It is hard to wake up during deep sleep, and if one is woken during deep sleep, one will likely experience brain fog and feel tired or dizzy.

Stages of Sleep

In understanding sleep, it is necessary to understand the stages of sleep the body goes through. During sleep, the body goes through three non-rapid eye movement (NREM) stages of sleep, followed by one rapid eye movement (REM) stage.

These four stages generally take ninety to one hundred and twenty-two minutes to cycle through fully before starting again. A typical adult will have between four to six cycles per night. The earlier half of the night is spent in NREM sleep, but as the night goes on, more time is spent in REM sleep.

Sleep Cycles  

The sleep cycle occurs four to six times in the average adult at night. This cycle consists of four sleep stages, three of NREM sleep and one of REM sleep.

NREM sleep consists of three stages, the first two stages consist of light sleep, and the third is deep sleep. REM sleep consists of one stage, usually when vivid dreaming is present.

Non-REM Sleep

Stage 1

In this stage of light sleep, functions such as heart rate, respiration, and eye movements begin to slow, muscles start to relax and may twitch a few times, and brain waves begin to slow down. This is a brief, almost drowsy stage where the body transitions into a deeper sleep. This stage typically lasts one to seven minutes.  

Stage 2

In the second stage of light sleep, heart rate and respiration slow even more, muscles relax, and core body temperature drops. Brain waves show a new pattern, and eye movement stops. Brain activity continues to become slower but with some short bursts of activity that act to help prevent being woken by environmental stimuli.

This stage makes up around 50% of the total sleep cycle and lasts longer in each cycle as the night progresses. The body will fall into stage two of NREM sleep more than any other stage throughout the night, typically lasting ten to twenty-five minutes during the first cycle and progressively getting longer.  

Stage 3: Deep Sleep

Stage three of the sleep cycle is when deep sleep occurs. In this stage, it is hard to wake up. The body continues to relax even further, with heart rate, respiration rate and muscle tone relaxing more.

Brain activity in deep sleep has an “identifiable pattern” of brain waves called delta waves. This is why stage three and deep sleep are called delta and slow-wave sleep.

This stage typically lasts twenty to forty minutes, with the body spending the most time earlier at night. As the night and sleep progresses, the amount of time spent in deep sleep shortens, and the amount of time spent in REM sleep increases.

REM Sleep

Stage 4

The final stage of the sleep cycle is rapid eye movement (REM) sleep. During REM sleep, the eyes move quickly beneath the eyelids, and brain activity increases to similar levels of someone awake. The body experiences atonia, where the muscles go through temporary paralysis. However, the exception is the eyes and the respiratory muscles.

REM sleep is when vivid dreams usually happen, correlated to increased brain activity. However, dreams can occur in other stages of sleep; they are less common and vivid.    

Typically the body doesn’t enter REM sleep until the body has already been asleep for ninety minutes. The stage lasts around ten minutes early on in the night, but as the night progresses, the REM stage can last up to sixty minutes in the second half of the night.  

The Importance of Deep Sleep

Although getting enough sleep is generally important, with the typical adult needing between seven to nine hours of sleep each night, deep sleep has particular physical and mental benefits that the other stages don’t have.

The physical benefits of deep sleep are vast. In a night of deep sleep, the pituitary gland releases human growth hormone, which is critical for the body to build and repair muscles, bones, cells, and tissue and replenish energy stores.

Deep sleep helps to promote a strong immune system. It can help to regulate glucose metabolism, blood pressure reaches its lowest point of the day, and there is an increase in blood supply to the muscles.

Deep sleep also benefits the body cognitively and is critical for cognitive function and memory. It plays a role in learning language, performing and learning motor skills and brain development. And even allows the brain to prepare synapses to take in new information and adapt to changing environments daily.

How Much Deep Sleep is Necessary? 

To determine the amount of necessary deep sleep, it is critical to determine how much overall sleep an individual needs. Typically, the average adult needs 7-9 hours of sleep; and 75% of this is NREM sleep, 25% is REM sleep. Roughly 13-23% of the total time asleep is time spent in deep sleep.

Ideally, the body regulates deep sleep, adjusting the time spent in deep sleep based on the rest and recovery it needs. During times of higher stress on the body or sleep deprivation, the body will need more deep sleep. During periods where naps may be more frequent, deep sleep during the night may be less.  

Sleep quality is important; however, as people age, they experience less deep sleep, which is replaced with more sleep in stage two of the sleep cycle. Younger people need more deep sleep as their bodies are still growing and developing.

What Happens When There Is Not Enough Deep Sleep

A lack of deep sleep can cause many conditions, physical and mental disorders, and sleep disorders, on top of just chronic fatigue.

Deep sleep plays a significant role in the memory formation process. So even with one night of sleep deprivation or poor quality sleep, one might struggle to remember information or learn.

Physically, not getting enough deep sleep harms the immune system. Making one more susceptible to catching illnesses, increased inflammation, musculature not fully recovered and less cell regeneration.

Deep sleep allows for “harmful waste products” to be eliminated from the brain. Not getting enough deep sleep can put one at greater risk for neurological conditions like Alzheimer’s and Parkinson’s.

Sleep deprivation or poor sleep quality harms hormonal changes that can lead to an increased appetite for often calorically dense. A lack of slow-wave sleep can play a part in insulin resistance, type 2 diabetes and heart disease.    

Sleep Disorders Connected to Deep Sleep

Who struggles to get enough deep sleep?

Anyone consistently getting less sleep than is recommended is likely not getting enough deep sleep. Someone who does shift work where their sleep time is not in sync with the natural sleep-wake rhythm will experience lower amounts of deep sleep.

Sleep disorders such as sleep apnea or insomnia can affect sleep cycles, increasing stage one sleep and decreasing slow-wave sleep. Other disorders, such as schizophrenia, Alzheimer’s disease and Parkinson’s disease, can hurt slow-wave sleep.

It has been shown that a damaged hippocampus, which is the brain's memory center, can negatively affect slow-wave sleep. The hippocampus sends memory signals crucial for creating slow delta waves in slow-wave sleep; if it is damaged, these signals aren’t created to produce the delta waves in deep sleep.    

Signs and Symptoms of Lack of Deep Sleep

Signs of insufficient deep sleep include constantly feeling tired and foggy, decreased alertness and ability, struggles to learn and memorize, and cravings for calorically dense foods.  

How to Measure Deep Sleep


If a person is constantly feeling exhausted, it might be worth testing how much sleep they are getting and, more specifically, how much deep sleep they have.

Wearable technology can help people understand overall sleep patterns, how often they wake up and how restless they are.

These devices, however, do not accurately depict how much deep sleep a person is getting, and a doctor can recommend a sleep study called polysomnography (PSG). This test requires the person to sleep in the lab and be hooked to monitors measuring respiratory rate, oxygen levels, body movements, heart rate and brain waves. This will show the doctor if deep sleep is being reached during each sleep cycle.

Tips to Get Better Deep Sleep

tips to get better deep sleep

It is important to fall asleep consistently and get more deep sleep necessary for rest and rejuvenation.

Good Sleep Hygiene

Cleaning up sleep habits through creating a consistent sleep schedule with the same wake and sleep times can help the body fall asleep quickly and have a more consistent sleep, improving sleep quality.

Creating a bedtime routine for before bed can promote relaxation. Eliminating bright lights, screens and blue light, tv and computers, and replacing them with reading can help increase sleep quality.

Ensuring the sleep environment is cool and dark can help to regulate core body temperature and improve sleep quality.  

Regular Exercise

Regular exercise, at least twenty to thirty minutes of moderate exercise per day, can help to regulate the sleep cycle and improve deep sleep. Researchers have found that aerobic exercise can increase the amount of slow-wave sleep a person gets. 

Warm Bathing

Taking a warm bath before bed can have a positive impact on helping to prepare the body for slow-wave sleep. When having a warm bath, the heat spreads to the hands and feet, allowing core body temperature to cool down for sleeping, ensuring quality sleep.

Healthy Diet

Food and drink consumed before sleep plays an effect on sleep quality. It is important to avoid caffeine, alcohol and nicotine as they impact the body’s ability to fall asleep.

The American Academy of Sleep Medicine has also found that “what you eat can influence how you sleep.” Foods high in saturated fats negatively impact slow-wave sleep, but fiber-rich foods benefit deep sleep.

Binaural Beats

Binaural beats are created when two tones are played, one in each ear. These two different frequencies create a perceived third tone called the binaural beat. Delta wave binaural beats have the potential to “induce delta waves in the brain and therefore stage three sleep.”  



Deep Sleep: What It Is and How Much You Need

Stages of Sleep: What Happens in a Sleep Cycle | Sleep Foundation 

What you eat can influence your sleep: Daily fibre intake, saturated fat and sugar may impact sleep quality -- ScienceDaily.

Exercising for Better Sleep | Johns Hopkins Medicine

Sleep - Wikipedia 


The contents of this article are provided for informational purposes only and are not intended to substitute for professional medical advice, diagnosis, or treatment. It is always recommended to consult with a qualified healthcare provider before making any health-related changes or if you have any questions or concerns about your health. Anahana is not liable for any errors, omissions, or consequences that may occur from the use of the information provided.