When you sleep, your body and brain go through physiological changes. According to the changes, your sleep cycle is classified into two major stages: The stage of Non-Rapid Eye Movement (NREM) and the Rapid Eye Movement (REM) stage.

The NREM stage makes up about 80% of your sleep hours and lasts for 4 to 6 hours and forms the important and major part of the sleep cycle. The REM stage lasts for about 2 hours.

NREM sleep and REM sleep are both important. Both are required to rest the body fully. Without one, the functions of the other would be ineffective.

The EEG and biological changes that take place in a particular sequence during the NREM stages contribute to the proper hours of good sleep, the need, and the importance of which cannot be underestimated.

What is NREM sleep? Definition

The Non-rapid eye movement (NREM) stage is a state of quiet sleep that repeatedly occurs during your sleeping hours and alternates with REM sleep. It is characterized by delta wave brain activity, which is an activity with a high amplitude electrical rhythm of the brain with a low frequency.

There is little or no dreaming and a low level of autonomic physiological activity. The muscles have little tone, your breathing is regular and the use of energy by the brain is minimal.

What happens during NREM sleep?

During NREM sleep, you do not dream much and it is a stage of quiet sleep. This stage consists of four sub-stages each being deeper than the previous one: Stages 1, 2, 3, and the fourth stage of this sleep, which is known as delta sleep or slow-wave sleep.

In NREM sleep, as the stages of sleep progress, the brain waves become slower and more synchronized.

On EEG, the brain waves during NREM sleep are typically slow and of high voltage. The breathing is slow and the heart rate and blood pressure are, low. There are no eye movements as seen in REM sleep.

Importance of NREM sleep

This NREM classification is made in order to identify the characteristic physiological changes that occur in the brain and body during each of the stages. Why these brain and body changes happen during each of the stages is still a biological mystery.

However, one thing is certain and that is, during NREM sleep, the body repairs and builds tissues, bone and muscle, makes your immune system stronger and restores energy levels.

NREM sleep is crucial to retain memory and acquiring and refining our motor skills.

Stages of NREM sleep

As mentioned above, NREM sleep is divided into four stages. This staging was done way back in 1968 in the Rechtschaffen and Kales (R&K) standardization.

This staging has been revised and The American Academy of Sleep Medicine in 2007 has reduced and clubbed these four stages into three stages.  

Each stage can last from five to 15 minutes. Stages 2 and 3 repeat in reverse before REM sleep sets in.

NREM Stage 1– Transition to sleep

Stage 1 is the beginning of your sleep cycle. It is light sleep, which lasts briefly for 5 to 10 minutes. It is a transition from wakefulness to sleep. You are easily awakened in this stage of sleep and when awakened you may feel that you were not asleep at all.

During this stage, muscle activity slows down and the eyes move slowly under the closed eyelids.

In this stage, you may experience sudden jerky movements of some muscles, giving a sensation of falling down from the bed. These are called myoclonic jerks and are caused by the spontaneous stimulation of the motor centers of the brain.

On EEG, this stage produces theta waves, which are high-amplitude slow waves.

NREM Stage 2 – Stage of light sleep

This stage is the beginning of true sleep, which lasts for 10 to 25 minutes. In this stage, there are no eye movements, the heart rate slows down and the body temperature decreases.

You spend about 50 % of your sleeping time in this stage. Brain waves become slower indicating slower brain activity.

However, there will be brief bursts of rapid and rhythmic brain activity shown by sleep spindles on EEG. Sleep talking usually occurs during stages 1 and 2.

NREM Stage 3 (and Stage 4) – Stage of deep sleep

This is a stage of transition from light sleep to deep sleep with the 4th stage being more intense. It is difficult to awaken a person from this stage and if awakened the person is groggy and disoriented for a few minutes.

Blood is directed away from the brain to the muscles to restore physical energy. Deep slow brain waves called delta waves emerge during this stage combined with fast waves. During this stage, your blood pressure and heart rate decline.

The EEG shows high amplitude low-frequency delta waves. This is a stage of deep sleep that lasts for about 30 minutes.

Sleepwalking and bedwetting occur toward the end of this stage. This stage is important because it is the satisfying sleep in this stage that makes you feel refreshed in the morning.

Though NREM is a dreamless sleep, it is found that you do dream in the last stage of this sleep, though not as commonly as in REM sleep.

The dreams of this deep sleep stage tend to be disconnected, less clear, less memorable, and tend to have a rather thought-like nature.