The Four Stages of Sleep (NREM and REM Sleep Cycles)

Stages of sleep
The four stages of sleep are marked by variations in brain activity. Stephen Simpson/Stone/Getty Images

You've probably heard that you progress through a series of stages as you sleep, but what exactly does that mean? Sleep is sleep, right? In reality, there is still a lot going on inside your head while you slumber, and it is the activity in your brain that marks these different sleep stages.

It was the electroencephalograph (EEG) invention that allowed scientists to study sleep in ways that were not previously possible.


There are two main types of sleep:

  1. Non-Rapid Eye Movement (NREM) Sleep (also known as quiet sleep
  2. Rapid Eye Movement (REM) Sleep (also known as active sleep or paradoxical sleep

The Beginnings of Sleep



Another very common event during this period is known as a myoclonic jerk. If you have ever startled suddenly for seemingly no reason at all, then you have experienced this phenomenon. While it might seem unusual, these myoclonic jerks are actually quite common.

Previously, experts divided sleep into five different stages.

Fairly recently, however, stages 3 and 4 were combined so that there are now 3 NREM stages and a REM stage of sleep.

NREM Stage 1



NREM Stage 2

During stage 2 sleep:

  • People become less aware of their surroundings
  • Body temperature drops
  • Breathing and heart rate become more regular

Stage 2 is the second stage of sleep and lasts for approximately 20 minutes. The brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles. Body temperature starts to decrease and heart rate begins to slow. According to the American Sleep Foundation, people spend approximately 50 percent of their total sleep in this stage.

NREM Stage 3

During stage 3 sleep:

  • Muscles relax
  • Blood pressure and breathing rate drop
  • Deepest sleep occurs

This stage was previously divided into stages three and four. Deep, slow brain waves known as delta waves begin to emerge during stage 3 sleep. This stage is also sometimes referred to as delta sleep.

During this stage, people become less responsive and noises and activity in the environment may fail to generate a response. It also acts as a transitional period between light sleep and a very deep sleep.

Older studies suggested that bed-wetting was most likely to occur during this deep stage of sleep, but some more recent evidence suggests that such bed-wetting can also occur at other stages. Sleepwalking also tends to occur most often during the deep sleep of this stage.

REM Sleep

During REM sleep:

  • The brain becomes more active
  • Body becomes relaxed and immobilized
  • Dreams occur
  • Eyes move rapidly

Most dreaming occurs during the fourth stage of sleep, known as rapid eye movement (REM) sleep. REM sleep is characterized by eye movement, increased respiration rate and increased brain activity. The American Sleep Foundation suggests that people spend approximately 20 percent of their total sleep in this stage.

REM sleep is also referred to as paradoxical sleep because while the brain and other body systems become more active, muscles become more relaxed. Dreaming occurs due to increased brain activity, but voluntary muscles become immobilized.

The Sequence of Sleep Stages

It is important to realize that sleep does not progress through these stages in sequence. Sleep begins in stage 1 and progresses into stages 2, and 3. After stage 3 sleep, stage 2 sleep is repeated  before entering REM sleep. Once REM sleep is over, the body usually returns to stage 2 sleep. Sleep cycles through these stages approximately four or five times throughout the night.


While sleep is often thought of as a passive process, research has shown that the brain is actually quite active during different stages of sleep. Sleep plays an important role in a number of processes including memory consolidation and brain cleanup.

Learn more about some of the surprising ways that sleep affects the brain.


American Sleep Association. (). What is sleep? Retrieved from

Cendron, M. (1999). Primary nocturnal enuresis: Current concepts. American Family Physician, 59(5), 1205-1214.

National Sleep Foundation. (n.d). What happens when you sleep? Retrieved from

Pressman, M.R. (2007). Factors that predispose, prime, and precipitate NREM parasomnia in adults: Clinical and forensic implications. Sleep Medicine Reviews, 11(1), 5-30.

Purves, D., Augustine, G.J., Fitzpatrich, D., et al. (2001). Neuroscience, 2nd edition. NCBI Bookshelf. Sunderland, MA: Sinauer Associates. Retrieved from

Continue Reading