Stages of Sleep: An In-Depth Guide to How Sleep Cycles Work

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A healthy sleep cycle consists of multiple stages that you’ll cycle through several times. Each stage has a different duration, and for some of the stages, their duration will change throughout the course of the night.

Your sleep cycle as a whole plays an important role in brain and body recovery. Getting at least 7 hours of sleep each night and spending adequate time in each stage is essential for waking up refreshed in the morning. However, factors such as age and physical health may interfere with your sleep cycle and prevent you from getting the rest you need.

We’ll take an in-depth look at each stage of your sleep cycle. We’ll also discuss the importance of your sleep cycle and offer tips for optimizing your nightly sleep.

The four stages of sleep fall into two categories. The first three stages are considered non-rapid eye movement sleep (NREM), while the fourth and final stage is rapid eye movement (REM). As the name suggests, REM sleep is distinguished by erratic eye movements behind closed eyelids, but there are other key differences between the REM and NREM stages.

The general breakdown for your sleep cycle stages is as follows:

Some classifications also include a stage simply known as “wake.” This stage is defined as the period immediately prior to sleep when you become drowsy and your eyelids involuntarily close. When you are awake, beta brain waves promote feelings of arousal whereas alpha waves make you feel relaxed. The “wake” stage officially kicks off when more than 50% of your brain wave activity consists of alpha waves.

Stage 1/NREM 1

The NREM 1 stage marks the transition between wakefulness and sleep, when alpha waves gradually give way to low-amplitude mixed frequency brain activity. As this stage begins, heartbeat and breathing rates decrease. Your eye movements become slower and your muscles relax, though some people experience slight muscle twitches called hypnic jerks during this stage.

The shortest sleep stage, NREM1 typically lasts one to five minutes per cycle and represents about 5% of your overall sleep. NREM 1 is also the stage with the lightest sleep, meaning your sleep can be easily disrupted.

Stage 2/NREM 2

NREM 2 is also considered a light sleep stage, occurring before you transition into deep sleep. Your heartbeat, breathing rate, and muscle activity all continue to decrease. Additionally, your body temperature drops and your eye movements cease. This is the longest of the four sleep stages, representing about 50% of your total sleep time. Your initial NREM 2 stage lasts about 25 minutes or so, but its duration increases with subsequent sleep cycles.

During NREM 2, your brain may display brief bursts of activity known as sleep spindles. Usually lasting no longer than three seconds, spindles are thought to help your body ignore external stimuli and allow you to progress into deeper sleep. The NREM 2 stage is also when K-complexes occur. K-complexes are singular delta wave bursts that last about a second. As you fully transition into deeper sleep, your brain activity consists entirely of delta waves.

If you enjoy taking naps during the day, most sleep experts suggest timing your snooze to end during the NREM 1 or NREM 2 stages. Waking up during these stages is associated with more alertness. Trying to wake up during the NREM 3 or REM stages can make you feel overly groggy upon waking.

Stage 3/NREM 3

NREM 3 marks the beginning of deep sleep. During this stage, your heartbeat and breathing rate fall to their lowest levels of the sleep cycle. Your muscles also become completely relaxed. The stage is longer during the first half of the night. Brain activity consists of delta waves with a slow frequency. For this reason, the NREM 3 stage is often referred to as slow-wave sleep.

People in the NREM 3 stage cannot be easily awakened, and may even sleep through noises of 100 decibels or louder. Waking up during this stage can cause a condition known as sleep inertia, which is defined by mental fogginess and difficulty concentrating. You may experience sleep inertia impairments for up to an hour after waking.

Historically, the NREM 3 stage was divided into two separate stages known as N3 and N4. This changed in 2007 when the American Academy of Sleep Medicine changed the classification and redefined N3 and N4 as a singular stage. Some authorities continue to use the older system, but the four-stage sleep cycle has been widely accepted.

Stage 4/REM

REM sleep represents a dramatic departure from the three preceding stages. Although your body is completely at rest, electroencephalogram (EEG) readings indicate brain activity is similar to that of an awake person. Heart rate and blood pressure also increase, and your eyes also move erratically from side to side behind their eyelids.

REM sleep is when most of your dreams occur. Your body and limbs undergo a temporary paralysis during this stage – research suggests this restricts you from physically acting out on your dreams. The REM stage is associated with memory consolidation and creative problem-solving.

The first REM stage occurs about 90 minutes after you fall asleep and lasts 10 minutes. The duration of this stage increases during the night, eventually lasting up to an hour.

Healthy sleep plays an important role in physical recovery. During the NREM 3 stage, your body repairs its tissue, builds bone and muscle, and recharges the immune system. Some studies have also suggested a link between cardiovascular disease and sleep disorders like insomnia. Researchers believe this relationship is due to the way insomnia stimulates activity in the sympathetic nervous system. Over time, too much of this activity can lead to heart problems.

Sleep is crucial for your immune system, as well. Activity in the sympathetic nervous system decreases during NREM sleep and increases again once REM sleep begins. When you don’t get adequate sleep, your sympathetic nervous system may be too active during the day. This can lower your immune system’s defenses and leave you more susceptible to getting sick.

During sleep, most body functions remain active. Sleep restores cellular damage, repairs tissues, fights infections, and promotes brain function – but it doesn’t happen all at once. Your body tackles this important work in stages.

Dr. Michael Breus

There are a few factors that can affect your sleep cycle and how long you spend in different stages. These include:

• Age: Your sleep cycle will evolve as you age, and this process begins in infancy. Newborns spend a large percentage of their sleep in the REM stage. As the duration of their sleep intervals become longer, REM sleep constitutes less of their overall sleep cycle. You gradually spend less time in the REM stage as you age. Researchers have noticed a significant change in REM sleep duration between young adulthood and middle adulthood.
• Alcohol Consumption: Many people mistakenly believe that consuming alcohol before bed can help them sleep. While alcohol has sedative effects that can induce feelings of tiredness, studies show drinking can suppress REM sleep during the first half of the night. This can lead to a “REM rebound” during the second half of the night.
• Sleep Disorder: Some sleep disorders cause nighttime awakenings that disrupt the sleep cycle and prevent you from reaching certain stages. These disorders include sleep apnea and restless legs syndrome.

How Can You Make Your Sleep Cycle Healthier?

Getting a full night’s rest is crucial for reaping the benefits of a healthy sleep cycle. For some people, this is easier said than done. A good first step for many is to improve their sleep hygiene, which is a collective term for habits and behaviors that impact sleep. Guidelines for healthy sleep hygiene include:

• Going to bed and waking up at the same times each day, even on weekends
• Exercising up to 30 minutes during the day, but refraining from vigorous workouts close to bedtime
• Abstaining from caffeine, nicotine, and alcoholic beverages in the late afternoon or evening
• Don’t force yourself to sleep if you wake up during the night. Instead, get up for a few minutes and engage in a relaxing activity like reading or listening to music
• Maintain a sleep-friendly bedroom that is dark, cool, and quiet

If you struggle to get enough sleep each night, there might be more to your situation than problems with sleep hygiene. Talk to a doctor about your specific problems. They can provide additional tips and either evaluate you for a sleep disorder or refer you to someone who is qualified to do so.

About The Author

Afy Okoye

Staff Writer, Sleep Health

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Afy is a writer and creative strategist in San Francisco with a master’s degree in international health policy from the London School of Economics. She has written for VeryWell Health, BlackDoctor.org, and Paste magazine and edited peer-reviewed journal manuscripts for Elsevier. Afy says her work with The Sleep Doctor is anything but “sleepy.” She enjoys the opportunity to learn new information and share knowledge that gives people the power to make better choices. Afy also likes to read non-fiction, do creative writing, and travel solo.

• POSITION: Side Sleeper
• TEMPERATURE: Hot Sleeper
• CHRONOTYPE: Bear

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