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.
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.
This article discusses the four stages of sleep. It also explains what happens during each sleep stage and what can hinder sleep.
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 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. The brain is still relatively active and producing high amplitude theta waves, which are slow brainwaves occurring primarily in the brain’s frontal lobe.
NREM Stage 2
People spend about half 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 environmental noises or activity may fail to wake the sleeping person.
Sleepwalking typically occurs during NREM stage 3 sleep. It is more common in the early part of your night’s sleep. Children and young adults are more likely to sleepwalk than older adults.
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. Getting enough NREM stage 3 sleep makes you feel refreshed the next day.
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.
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.
Dreaming Sleep and Sleep Cycles
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:
- Sleep begins with NREM stage 1 sleep.
- NREM stage 1 progresses into NREM stage 2.
- NREM stage 2 is followed by NREM stage 3.
- NREM stage 2 is then repeated.
- Finally, you are in REM sleep.
Once REM sleep is over, the body usually returns to NREM stage 2 before beginning the cycle.
Time spent in each stage changes throughout the night as the cycle repeats (about four to five times total).
Sleep architecture refers to the cycles and stages a person experiences at night. A sleep specialist may show you this information on what’s known as a hypnogram—a graph produced by an EEG.
How Long Is a Sleep Cycle?
According to the National Sleep Foundation, a full sleep cycle is generally around 90 minutes long.
Factors That Affect Your Sleep 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.
Several issues can interrupt your sleep cycles. This may happen occasionally or chronically, depending on which one is at play.
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 (an intense 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 or anxiety
- Other health conditions including Alzheimer’s disease, Parkinson’s disease, and obesity
- 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 various ways, potentially having 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:
- High blood pressure
- Heart disease
- Reduced quality of life
Tips For a Healthier Sleep Cycle
According to the Centers for Disease Control and Prevention, 1 in 3 adults in the US reports not getting enough sleep. There are things everyone can try to help improve the quality and quantity of sleep.
- Limit electronics before bed.
- Try to get at least half an hour of natural sunlight per day.
- Go to bed and wake up at roughly the same time each day.
- Get some exercise each day.
- Don’t eat a heavy meal before bed.
- Avoid alcohol.
- Keep your room cool and dark.
- Get adequate sleep. The longer you sleep, the more REM sleep you will get.
If you practice good sleep hygiene, you can often improve the quantity and quality of your sleep. If you still are not getting sufficient sleep after trying the above tips for at least a week, see a healthcare professional to assess if you need other assistance, such as medication or a sleep apnea device.
As your body progresses through the four sleep cycle stages—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 various 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
The American Academy of Sleep Medicine and the Sleep Research Society recommend that adults get seven or more hours of sleep per night. But it’s important to get uninterrupted, quality sleep that allows your body to benefit from these four stages.
If you experience any of the following, make an appointment to see a 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
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