What is Deep Sleep?

By: SleepScore Labs  |  June 10th, 2017

What is Deep Sleep? 

You’ve probably heard of non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep, but what makes one different from the other? Each has a vital role in helping you get optimal sleep and supporting daytime functioning, and they happen at various times in the night in specific succession. 

During sleep, the NREM and REM sleep stages take turns as they move in cycles. Deep sleep is  happens during the third stage of NREM sleep. This sleep stage’s primary role is to help you get refreshing and restorative sleep

This article will focus on deep sleep, what happens during this phase, how it’s connected with the other sleep stages, the benefits of deep sleep, and tips for getting more deep sleep.

What Does Deep Sleep Mean?

Deep sleep is also known as NREM-3or slow-wave sleep. During this stage, the brain’s neurons fire in unison, and as a result your brain activity shows as high amplitude, low-frequency waves (AKA., slow waves).

This sleep stage helps the body and mind refresh, restore, and revitalize itself. 

Additionally, deep sleep tends to take place during the first half of the night and takes up to one-third of a person’s sleep duration. But as the night goes by, a person starts to experience less deep sleep as REM sleep takes over. 

What Happens During Deep Sleep? 

A person falls into heavy sleep during deep sleep, and they’re usually the hardest to wake up when they’re in this sleep stage. 

It’s the most difficult stage to wake from because a person is the most disconnected from their environment. Interestingly, experts state that even loud noises, over 100 decibels, may not wake up a person in this sleep stage. And if something or someone wakes them up, they’ll feel groggy and sluggish as they try to get out of bed. This experience is known as sleep inertia. And the person may feel it for about 30 minutes to an hour. 

Deep sleep is a highly restorative phase. The body experiences slow-wave activity about an hour into sleep. This process happens because this is the period the body and mind are the most tired and need the restorative process. 

During this stage, a person’s heart rate and breathing rate drop to their lowest, and muscles relax. 

Deep sleep stage is also associated with no or slow eye movement, memory consolidation, and the release of growth hormones.

The body heals and repairs itself as this sleep phase allows your body to replenish the cells and rejuvenate the immune system.

However, as people age, their bodies experience more stage 2 sleep and less deep sleep. 

What Are the Other Stages of Sleep?

Sleep is an indispensable part of living that helps our body and mind function round the clock. It occurs in four stages: NREM-1, NREM-2, NREM-3, and REM sleep. 

1. Wake 

This phase precedes the first sleep stage  in NREM. A person’s eye may either be open or closed at this stage. 

When it’s open, the brain starts experiencing alpha and beta waves activity. But beta waves are dominant. Alpha waves become dominant as the person dozes off and the eyes close.

2. NREM-1 sleep

Stage NREM-1 sleep is the first sleep stage in NREM sleep, and it’s described as the transitional light sleep stage. 

A person falls into this sleep stage when moving from wakefulness to sleep. 

This stage is the lightest sleep stage is the stage of sleep we spend the least amount of during the night.

It’s easy to wake a person up during this phase, and they might wake up not knowing that they were even asleep. During this stage, eye movements, heart rate, and breathing rate slowly fall, and muscles start to relax and may also twitch.

3. NREM-2 sleep 

Although stage NREM-2 sleep is also a light sleep stage. Here, the body prepares itself to enter deep sleep. 

Heart rate, body temperature, and breathing rate drop even further. The muscles also go into a relaxed state, and there’s no eye movement.

 It accounts for 50 percent of total sleep. 

4. NREM-3 sleep

Stage NREM-3 sleep is also known as deep sleep or slow-wave sleep. It is the deepest sleep stage and the most important for recovery and restorative sleep. 

During this sleep stage, the body experiences no eye movements. Heart rate, blood pressure, and breathing rate drop to their lowest, muscles are completely relaxed, and delta brain wave activity occurs. 

5. REM sleep

Also known as paradoxical sleep or desynchronized sleep, REM sleep is the last sleep stage, marking the end of a typical sleep cycle. 

Dreaming happens predominantly in this sleep stage. This sleep stage is also crucial for emotion regulation and memory consolidation. 

It happens about 90 minutes into sleep and takes up about 20 to 25 percent of total time spent in sleep. However, the last REM cycle may go on for up to an hour.

The brain is active during this stage. The American Sleep Association says that a person experiences dreams, limited or no muscle activity to prevent them from acting out their dreams, and rapid eye movements.

Why is Deep Sleep Important?

The deep sleep stage’s vital function is to help the body and mind repair, grow, refresh and restore itself

This sleep stage also supports the improved functioning of the immune system. 

A 2020 study suggests that deep sleep may benefit memory consolidation, just like REM sleep.

Additionally, deep sleep may help maintain positive emotional states and allow the mind to recharge. According to a 2019 study, deep sleep may help a person feel calm, relieve their anxiety symptoms, and “reset the anxious brain.”

Deep sleep may also help the brain learn and process information fast and retain all a person mentally takes in during the day. According to a 2017 study, deep sleep supports the brain’s learning ability and helps a person consolidate what they learn. 

What is the Difference Between REM Sleep and Deep Sleep?

In REM sleep, the brain becomes active as though it’s in a wakeful state. Breathing rate becomes erratic, heart rate and blood pressure rise, but there are no muscle movements. 

This sleep stage plays a significant role in regulating emotions and consolidating memories. A person may encounter four to five REM sleep cycles—each for about 90 to 120 minutes—during sleep, but longer REM sleep happens in the second half of sleep. 

Generally, REM sleep is associated with increased brain activity, dreaming, no muscle activity, and rapid eye movements.

In contrast, deep sleep is marked by slow brain wave activity, no eye movements, low heart rate and breathing rate, and relaxed muscles. 

What is a Normal Amount of Deep Sleep?

NREM sleep accounts for up to 80 percent of sleep duration, while REM sleep takes 20 to 25 percent of sleep time. 

Deep sleep takes up about 13 to 23 percent of a person’s sleep time. However, as a person grows older, they spend less time in deep sleep and more time in stage 2 sleep. 

What Happens If You’re Not Getting Enough Deep Sleep?

Deep sleep supports the body’s restorative process. When you don’t get enough deep sleep, you may notice that your daytime functioning slows down. 

Some of the signs you may experience include:

  1. Difficulty processing information
  2. Poor energy levels
  3. Learning difficulties
  4. Fatigue
  5. Poor attention
  6. Daytime sleepiness
  7. Slow response time
  8. Sluggishness

Disorders of arousal are also associated with partial arousal from deep sleep and some of them are bed-wetting, sleepwalking, sexsomnia, and night terrors.

Tips for Getting More Deep Sleep

According to the American Sleep Association, the most effective way to get more deep sleep is to get more sleep at night.

Generally, an adult needs seven or more hours of sleep. But this sleep requirement depends on individual factors like their health status, activity levels, and the daily stressors they expose themselves to. 

You can increase your total sleep time by following these general sleep hygiene practices:

  1. Avoid alcohol and caffeine during the evening time.
  2. Limit exposure to blue light from electronic devices.
  3. Maintain a consistent sleep-wake time.
  4. Make your sleep environment cool, dark, comfortable, and tidy.
  5. Follow a routine bedtime ritual that helps your body relax.
  6. Invest in more quality comfy pillows, a mattress, and pajamas.
  7. Avoid eating heavy meals at night.

Key Takeaways: What is Deep Sleep?

Deep sleep is a crucial sleep stage that helps the body and mind recharge and refresh. It’s also essential for the optimal functioning of a person’s body, brain, and emotions. 

Try to prioritize engaging in healthy sleep habits that support your sleep health to have more deep sleep at night. You can also invest in sleep aid devices like sleep masks, gravity blankets, and blue-light-blocking glasses to help improve your sleep. 


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