The 4 Stages of Sleep

As you sleep, your brain cycles through four stages of sleep. The first three are considered non-rapid eye movement (NREM) sleep, also known as quiet sleep. The fourth is rapid eye movement (REM) sleep, also known as active sleep.

In stage 1, you transition from wakefulness to sleep. In stage 2 you enter light sleep. By stage 3 you are in a deep sleep. Stage 4 REM, known as paradoxical sleep, is when the body's immobilized and dreams occur.

Each sleep stage 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 discusses the four stages of sleep. It also explains what happens during each sleep stage and what can hinder your sleep.

Falling 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 been startled suddenly for seemingly no reason at all, then you have experienced this.

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 slow 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 brain's frontal lobe.

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 repair, restore, and reset 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

During this deep sleep stage, 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.

Stage 4: REM Sleep

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

During REM sleep, 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.

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

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

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

What Happens When Sleep Stages Are Altered

Not spending enough time in each sleep stage or properly cycling through the stages of sleep can affect you in a variety of ways, having potentially short-term and long-term consequences.

A few examples of issues that can arise from a disrupted sleep cycle include problems with:

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

People with a disrupted sleep cycle are also at greater risk for:

• Pain
• Inflammation
• High blood pressure
• Heart disease
• Overweight/obesity
• Diabetes
• Reduced quality of life

Summary

As your body progresses through the four stages of the sleep cycle—stages 1 through 3 (non-rapid eye movement, or NREM) and stage 4 (rapid eye movement, or REM), it transitions through different biological processes that affect your temperature, breathing, cells, and muscles. All the while, your brain is busy forming, organizing, and storing memories.

The sleep cycle follows a specific pattern, but that can be interrupted because of variety of habits, health conditions, and even older age.

Over time, not getting enough sleep and not cycling through the four stages can cause physical and mental health issues.

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