Many conversations about sleep focus on the total hours slept at night. While sleep quantity is important, it doesn’t tell the whole story because not all sleep is the same.
Instead, the brain and body progress through different kinds of sleep that are categorized as rapid eye movement (REM) and non-REM (NREM) sleep. NREM sleep includes three stages and helps make sleep refreshing and restorative.
Knowing the facts about NREM sleep, including what it is and why it affects overall health, are integral to understanding sleep science and how to improve your sleep quality.
Understanding NREM Sleep
Non-rapid eye movement (NREM) sleep refers to specific parts of the sleep cycle. There are three stages of NREM sleep that occur multiple times during the night.
REM and NREM Sleep
Sleep can be divided into REM and non-REM sleep. Each serves specific restorative functions for the body and the brain, and both are crucial to a full night of sleep.
NREM sleep involves a reduced heart rate, lower blood pressure, and slower breathing than REM sleep. In REM sleep, brain activity picks up, which is why it is associated with vivid dreaming. Dreaming can happen in NREM sleep, but it is less vivid than in REM sleep.
REM stands for rapid eye movement because the eyes move quickly during this stage of sleep. In non-REM sleep, the eyes generally do not move.
On a normal night, a person goes through approximately four to six sleep cycles, each of which includes both REM and non-REM sleep. Earlier in the night, a greater percentage of each cycle is spent in NREM sleep. Closer to morning, REM sleep stages get longer. Overall, as much as 80% of the night is spent in NREM sleep.
Typically, most sleep when napping is NREM sleep.
What Happens During NREM Sleep?
NREM sleep is a chance for the brain and body to relax, which happens during three progressive stages.
The Stages of NREM Sleep
There are three stages of NREM sleep: stages 1, 2, and 3, which may also be called N1, N2, and N3.
Stage 1 is the first and lightest stage of sleep that happens right after falling asleep. It is the shortest of the three NREM stages. In this stage, brainwaves, breathing, and heartbeat all start to slow, and the muscles start to relax. Someone in stage 1 sleep is easy to wake up.
Stage 2 NREM sleep builds on stage 1. Breathing and heartbeat continue to slow down, and body temperature drops. Stage 2 occupies the largest percentage of total nightly sleep time of all the sleep stages. Although sleep in stage 2 is deeper than in stage 1, it is still considered light sleep.
The deepest sleep occurs in stage 3. Breathing and heartbeat are even slower than they are in stage 2. Stage 3 sleep is what people think of as high-quality sleep and may be referred to as deep sleep or slow-wave sleep. It’s hard to wake people up from this stage of sleep, and some people in stage 3 will sleep through even very loud noises.
After stage 3 sleep, a person moves into a period of REM sleep before starting the cycle again. A sleeping person will go through each of the NREM stages plus REM sleep around four to six times each night. The stages happen in the same order, but the amount of time spent in each stage can vary based on how long someone has been asleep.
For example, the amount of time spent in stage 2 becomes longer as the night progresses, but more stage 3 sleep occurs in the beginning of the night.
NREM Sleep Differences by Age
Sleep cycles and time spent in each stage vary at different points in someone’s life.
Newborns do not have the same NREM and REM stages of their sleep cycle. They have quiet sleep, indeterminate sleep, and active sleep, and, unlike in adults, sleep periods start with the active stage that is most similar to REM sleep. Newborn sleep switches to have NREM and REM stages at about three months, although their sleep cycles are much shorter than in adults.
Adolescents generally spend less time in deep NREM sleep because of hormonal changes related to puberty. Instead, they spend more time in stage 2 NREM sleep.
Young adults tend to have more stage 3 sleep. Older adults often sleep less than younger adults, and they generally spend less time in stage 3 NREM sleep.
The Importance of NREM Sleep
While few people question that the brain and body need sleep, the exact purposes of sleep are still not fully known. However, experts believe that NREM sleep is involved in crucial functions like building and repairing muscle and tissue, helping the immune system, reinforcing new memories, and getting the brain ready for the next day.
Physical Growth and Healing
NREM sleep is thought to have a variety of restorative functions, many of which concentrate in stage 3. Deep sleep is believed to help the body recover and reset. During NREM sleep, the body works to heal damaged tissue and build stronger muscles.
Immune System Support
Without enough sleep, the immune system doesn’t work as well, making you more likely to get sick. NREM sleep supports the immune system in two main ways.
During NREM sleep, certain immune responses ramp up to help fight infections and then slow back down during REM sleep. These infection-fighting responses may also signal to the brain that additional deep sleep is needed.
When not actively fighting an infection, NREM sleep may promote immune health by reducing unwanted stress on the nervous system. During NREM sleep, activity decreases in a part of the nervous system called the sympathetic nervous system, which regulates the “fight-or-flight” response.
When you are sleep-deprived, it may trigger too much sympathetic nervous system activity. This can interfere with the immune system and decrease the number of certain immune cells and how well they work.
Removing Brain Waste
It is normal for certain substances to build up in the brain over the course of the day as a result of brain activity. Studies suggest that NREM sleep enables a natural process of removing these waste substances from the brain.
Learning and Memory
During sleep, details of new experiences and information are turned into memories. This is a complex process since the brain takes in more information than it can realistically store. Evidence points to NREM sleep playing an important role in strengthening memories, especially about facts and similar details.
NREM sleep may also make it easier to take in new information. Specific elements of brain activity during stage 3 NREM sleep may also help form new connections between cells in the brain.
How is NREM Sleep Measured?
Sleep studies that use a technique called polysomnography are the way that time spent in each sleep stage is measured.
During a sleep study, sensors record precise information about brain waves and movement of the eyes and muscles. Experts have developed standardized formulas to interpret this data for every 30-second increment of sleep to determine how a person’s sleep stages unfold over the course of the night.
Many sleep trackers and apps use algorithms to try to estimate NREM and REM sleep, but they do not have the same sensors as those used during polysomnography. These devices have generally not been shown to reliably assess sleep stages.
Sleep Disorders and NREM Sleep
Certain sleep disorders are more likely to occur during NREM sleep. For example, sleepwalking, night terrors, and sleep talking happen while a person is in NREM sleep, especially stage 3 NREM sleep.
Because more time is spent in stage 3 early during sleep, these abnormal behaviors frequently arise in the first third of the night. They are believed to occur as a result of partial transitions out of deep sleep.
Tracking sleep stages can also help diagnose the cause of health problems. When a sleep study finds that a person’s sleep cycles are disrupted or off-kilter, it may be a sign of an underlying sleep disorder or other health condition. It can also be a reaction to a drug or medication.
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