The 4 Stages of Sleep

What's Happening During NREM and REM Sleep

As you sleep, your brain cycles through four stages of sleep.

  • Stages 1 to 3 are what's considered non-rapid eye movement (NREM) sleep, also known as quiet sleep.
  • Stage 4 is rapid eye movement (REM) sleep, also known as active sleep or paradoxical sleep.

Each has a unique function and role in maintaining your brain's overall cognitive performance. Some stages are also associated with physical repairs that keep you healthy and get you ready for the next day.

The entire sleep cycle repeats itself several times a night with every successive REM stage increasing in duration and depth of sleep.

This article covers the basics of the sleep cycle, what is happening when each sleep stage occurs, and what can affect your ability to move through these stages as you should.

4 stages of sleep

Verywell / JR Bee

Entering Sleep

Using an electroencephalogram (EEG), a non-invasive test that records brain activity, scientists are able to see how the brain engages in various mental activities as a person falls and is asleep.

During the earliest phases of sleep, you are still relatively awake and alert. At this time, the brain produces what are known as beta waves—small and fast brainwaves that mean the brain is active and engaged.

As the brain begins to relax and slow down, it lights up with alpha waves. During this transition into deep sleep, you may experience strange and vivid sensations, known as hypnagogic hallucinations.

Common examples of this phenomenon include the sensation of falling or of hearing someone call your name.

There's also the myoclonic jerk; if you have ever startled suddenly for seemingly no reason at all, then you have experienced this.

Aren't There 5 Stages of Sleep?

Sleep used to be divided into five different stages, but this was changed by the American Academy of Sleep Medicine (AASM) in 2007.

NREM Stage 1

The first stage of the sleep cycle is a transition period between wakefulness and sleep.

If you awaken someone during this stage, they might report that they were not really asleep.

During stage 1 sleep:

  • Your brain slows down
  • Your heartbeat, your eye movements, and your breathing slows with it
  • Your body relaxes and your muscles may twitch

This brief period of sleep lasts for around five to 10 minutes. At this time, the brain is still fairly active and producing high amplitude theta waves, which are slow brainwaves occurring mostly in the frontal lobe of the brain.

NREM Stage 2

According to the American Sleep Foundation, people spend approximately 50% of their total sleep time during NREM stage 2, which lasts for about 20 minutes per cycle.

During stage 2 sleep:

  • You become less aware of your surroundings
  • Your body temperature drops
  • Your eye movements stop
  • Your breathing and heart rate become more regular

The brain also begins to produce bursts of rapid, rhythmic brain wave activity, which are known as sleep spindles. They are thought to be a feature of memory consolidation—when your brain gathers, processes, and filters new memories you acquired the previous day.

While this is occurring, your body slows down in preparation for NREM stage 3 sleep and REM sleep—the deep sleep stages when the brain and body repairs, restores, and resets for the coming day.

NREM Stage 3

Deep, slow brain waves known as delta waves begin to emerge during NREM stage 3 sleep—a stage that is also referred to as delta sleep. This is a period of deep sleep where any noises or activity in the environment may fail to wake the sleeping person.

Getting enough NREM stage 3 sleep allows you to feel refreshed the next day.

During NREM stage 3 sleep:

  • Your muscles are completely relaxed
  • Your blood pressure drops and breathing slows
  • You progress into your deepest sleep

It is during this deep sleep stage that your body starts its physical repairs.

Meanwhile, your brain consolidates declarative memories—for example, general knowledge, facts or statistics, personal experiences, and other things you have learned.

REM Sleep

While your brain is aroused with mental activities during REM sleep, the fourth sleep stage, your voluntary muscles become immobilized.

It's in this stage that your brain's activity most closely resembles its activity during waking hours. However, your body is temporarily paralyzed—a good thing, as it prevents you from acting out your dreams.

REM sleep begins approximately 90 minutes after falling asleep. At this time:

  • Your brain lights up with activity
  • Your body is relaxed and immobilized
  • Your breathing is faster and irregular
  • Your eyes move rapidly
  • You dream

Like stage 3, memory consolidation also happens during REM sleep. However, it is thought that REM sleep is when emotions and emotional memories are processed and stored.

Your brain also uses this time to cement information into memory, making it an important stage for learning.

Repair Work in Progress

During deep sleep (stage 3 and REM), your cells repair and rebuild, and hormones are secreted to promote bone and muscle growth. Your body also uses deep sleep to strengthen your immunity so you can fight off illness and infection.

Sequence of Sleep Stages

It's important to realize that sleep does not progress through the four stages in perfect sequence.

When you have a full night of uninterrupted sleep, the stages progress as follows:

  1. Sleep begins with NREM stage 1 sleep.
  2. NREM stage 1 progresses into NREM stage 2.
  3. NREM stage 2 is followed by NREM stage 3.
  4. NREM stage 2 is then repeated.
  5. Finally, you are in REM sleep.

Once REM sleep is over, the body usually returns to NREM stage 2 before beginning the cycle all over again.

Time spent in each stage changes throughout the night as the cycle repeats (about four to five times total).

Sleep architecture refers to the exact cycles and stages a person experiences in a night. A sleep specialist may show you this information on what's known as a hypnogram—a graph produced by an EEG.

What Can Interrupt Your Cycle

Interrupted sleep is the term used to describe sleep that is not continuous throughout the night. When this happens, your sleep cycle can be disrupted. An in-progress sleep stage may be cut short and a cycle may repeat before finishing.

There are a number of issues that can interrupt your sleep cycles. Depending on which one is at play, this may happen occasionally or on a chronic basis.

Some factors that are associated with interrupted sleep and, therefore, may affect your sleep stages include:

  • Older age: Sleep naturally becomes lighter and you are more easily awoken.
  • Nocturia: Frequently waking up with the need to urinate
  • Sleep disorders, including obstructive sleep apnea (breathing that stops and starts during sleep) and restless leg syndrome (a strong sensation of needing to move the legs)
  • Pain: Difficulty falling or staying asleep due to acute or chronic pain conditions, like fibromyalgia
  • Mood disorders such as depression and bipolar disorder
  • Other health conditions, including Alzheimer's disease, Parkinson's disease, obesity, heart disease, and asthma
  • Lifestyle habits: Little/no exercise, cigarette smoking, excessive caffeine intake, excessive alcohol use

Any time you have trouble falling asleep or staying asleep at night, your sleep cycle will be affected.

Summary

As your body progresses through the four stages of the sleep cycle, it transitions through different biological processes that affect your temperature, your breathing, your cells, and your muscles.

All the while, your brain is busy forming, organizing, and storing memories.

Over time, not getting enough sleep and cycling through the four stages as you should can cause health issues and difficulty with the following:

  • Learning and focusing
  • Being creative
  • Making rational decisions
  • Solving problems
  • Recalling memories or information
  • Controlling your emotions or behaviors

A Word From Verywell

It's important not just to get seven to nine hours of sleep per night, but to ensure it's uninterrupted, quality sleep that allows your body to benefit from each of these four stages.

If you experience any of the following, make an appointment to see your healthcare provider, as you may not be getting the sleep you need:

  • You are having trouble falling or staying asleep at least three nights per week
  • You regularly wake up feeling unrested
  • Your daytime activities are affected by fatigue or mental alertness
  • You often need to take a nap to get through the day
  • A sleep partner has told you that you snore or gasp when you are asleep
  • Lack of sleep is affecting your mental wellbeing

Frequently Asked Questions

  • What is rapid eye movement sleep?

    Rapid eye movement (REM) sleep is the fourth stage of sleep. In this stage, brain activity increases to similar levels as when you're awake and causes vivid dreams. The brain temporarily paralyzes major muscles so that we cannot move while dreaming.

  • How much REM sleep do you need?

    There isn't a specific recommendation for how much REM sleep is needed. This is due to REM taking place in multiple intervals for varying lengths of time. However, most adults should try to get a total of at least seven to nine hours of sleep each night.

  • How long is each sleep stage?
    • NREM stage 1: Less than 10 minutes, begins right after falling asleep
    • NREM stage 2: Lasts anywhere from 30 to 60 minutes
    • NREM stage 3: Lasts between 20 to 40 minutes
    • REM sleep: About 10 minutes for the first period during sleep, then increasingly longer with later periods
  • How long is a sleep cycle?

    A full sleep cycle is generally around 90 to 110 minutes long. After one sleep cycle is complete, the process starts again from the beginning, and repeats until we wake up.

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15 Sources
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