Kendra Cherry, MS, is an author, educational consultant, and speaker focused on helping students learn about psychology.

Sanja Jelic, MD, is board-certified in sleep medicine, critical care medicine, pulmonary disease, and internal medicine.

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

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.

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

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.

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.

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.

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.

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.

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.

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:

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.

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.

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:

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.

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:

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

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:

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:

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.

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.

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.

Tossing and turning night over night can have a big impact on your quality of life. Our free guide can help you get the rest you need. Sign up for our newsletter and get it free.

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