What is NREM Sleep?

What is NREM Sleep?

In general, people consider sleep to be a state of rest. However, contrary to popular belief, sleep is indeed a highly active process. There are four stages of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep all of which are vital for optimal brain and body health.

Yet, a total of 75% to 80% of the entire sleep period is NREM sleep. With that in mind, we can conclude that non-rapid eye movement is of utmost importance. Namely, it is crucial for feeling rested in the morning, physical growth and repair, and memory consolidation. Therefore, let’s see what NREM sleep exactly and what happens during these stages.

What is NREM Sleep?

Non-rapid eye movement (NREM) sleep includes three stages known as stage 1, stage 2, and stage 3 NREM sleep. While each stage is characterized by distinctive mental processes, breathing, muscle activity, heartbeat, and brain waves slow down during all three stages. In addition, the first two stages of NREM sleep are also referred to as light sleep, whereas stage 3 is known as deep sleep. As the name itself suggests, it is the most difficult to wake up a person during stage 3 NREM sleep.

What Happens During NREM Sleep?

As already mentioned, each of the three NREM sleep stages is characterized by slowed-down bodily functions such as heartbeat, breathing, etc. which is essential for allowing reparative and restorative processes to occur. But now, let’s see what precisely happens during every NREM sleep stage.

Stage 1

Beginning with stage 1 NREM sleep, as you can guess it is the first stage of the sleep cycle and it is the transition between wakefulness. The first episode of this light stage typically lasts only a few minutes. During stage 1, breathing, heartbeat, brain waves, and eye movements start to decline. Also, motor movements begin to reduce even though hypnic jerks may occur which are actually muscle twitches.

Stage 2

During stage 2 NREM sleep, bodily functions continue to slow down while body temperature drops as well. And, half of the time of sleep per night is actually stage 2 sleep. The major distinctive features of this sleep stage are two specific types of brain activity, including:

  • Sleep Spindles – short bursts of brain activity that may last from 0.5 to 3 seconds. These brain activity bursts repeat every three to six seconds during this stage and in other stages from time to time. Sleep spindles are divided into slow and fast depending on the area of the brain they occur in and are vital for memory and learning.
  • K-Complexes – This unique type of brain activity is indeed a single sharp peak followed by a negative dip.


K-complex consists of a single sharp peak in electrical activity, followed immediately by a negative dip. Like sleep spindles, K-complexes may play a role in maintaining sleep by blocking out reactions to harmless sounds and lights. Somewhat paradoxically, K-complexes may also help wake you up if the brain perceives a stimulus as dangerous. K-complexes are thought to contribute to memory consolidation and neural maintenance to keep the brain running smoothly.

Stage 3

Your heartbeat, breathing, muscle activity, and brain waves are at their slowest during stage 3 sleep. This sleep stage is otherwise known as deep sleep because experts believe it to be the most critical stage for regenerating your body and brain.

The body releases growth hormones during this stage and carries out tissue, muscle, and bone repair. Researchers believe deep sleep helps regulate glucose metabolism, immune system functioning, hormone release, and memory.

The majority of brain waves during stage 3 NREM sleep are called delta waves. These are large waves with a relatively slow frequency of 1 to 4 Hertz. Stage 3 NREM sleep also displays so-called slow oscillations, which are even slower at 0.5 to 1 Hertz. Slow oscillations may help synchronize delta waves and spindle waves, which can also occur during deep sleep.

Together, delta waves and slow oscillations are referred to as slow wave activity, and this sleep stage is often called slow wave sleep.

People are less receptive to outside stimuli during deep sleep, so it can be difficult to awaken them. If you do manage to wake someone out of deep sleep, they may experience a period of grogginess called sleep inertia.

Most people obtain the bulk of their deep sleep at the beginning of the night. Without enough slow-wave sleep, you can wake up feeling unrefreshed. Thus, after a period of sleep deprivation, you compensate by reducing spindle activity and spending more time in deep sleep during the next sleep period. Deep sleep decreases across the lifespan, with young children obtaining more deep sleep and older adults typically receiving less deep sleep.

When to Talk to Your Doctor

Sleep deprivation can cause chronic health problems down the line, so it is important to monitor any changes to your sleep and wake patterns and aim for the recommended sleep times according to your age group.

If you are having trouble sleeping and you do not know why, ask your doctor. They can recommend sleep hygiene tips to help you sleep better, and they may be able to identify an underlying sleep disorder that could be interfering with NREM sleep.