The 4 Stages of Sleep And What Happens in Each
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Medically reviewed by Anju Goel, MD, MPH
Throughout the night, your body cycles through four stages of sleep. During these stages, your body’s brain waves, eye movements, and neurological activity varies. Together, the four stages last for about 90 minutes and your body cycles through these stages roughly four to six times per night.
The four stages include two main phases: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM is divided up into three sub-categories (N1, N2, and N3 sleep), bringing the total number of stages of sleep to four.
Keep reading to find out what happens in the brain and body during each stage of sleep, along with what factors tend to impact how well you sleep in each stage.
Related: How Much Sleep You Need, According to Experts
NREM Stage 1
This is a transitional stage of sleep and begins your sleep cycle. During this phase, you are in a light sleep, and easily awoken. You have probably been aware of this stage of sleep before, because it’s what you enter as you start to fall asleep.
Facts about this stage:
It lasts between one to five minutes.
It encompasses about 5% of your total night’s sleep.
Your body systems, including breathing, heartbeat, and eye movements begin to relax.
Muscle twitches occur occasionally during this phase, as your muscles start to relax.
Your brain waves begin to make the transition from a daytime to nighttime pattern.
NREM Stage 2
In stage two of NREM sleep, you begin to enter a deeper period of sleep. Your breathing and heartbeat slows down, your muscles soften and relax, and your eye movements decrease. You even begin to experience a drop in body temperature during this phase.
Facts about this stage:
It lasts 10-25 minutes.
You spend about 45% of your night in this stage.
Out of all the stages of sleep, you spend the most time in stage two.
The amount of time you spend in this stage lengthens as the night progresses.
Your overall brain wave activity decreases during this phase, but there are moment of electrical activity.
If you were given an ECG during this phase, sleep spindles (quick neurological bursts) and K-complexes (delta brain waves).
Stage two sleep is when you are most likely to grind your teeth.
Related: 11 Health Benefits of Sleep
NREM Stage 3
This is the deepest stage of sleep. When you are in this stage of sleep, you may be difficult to rouse, and you may be more likely to sleep through disturbances. This is the most restorative stage of sleep, and it’s needed to feel energetic and reinvigorated in the morning. If you are woken up during this stage of sleep, you are likely to feel most groggy and disoriented.
Facts about this stage:
It lasts about 20-40 minutes.
You spend about 25% of your night in this stage of sleep.
The longest periods of this stage occur during the first several sleep cycles.
During this stage, your breathing, heartbeat, and muscles are slow and relaxed.
You enter delta wave sleep during this time, or slow-wave sleep (SWS).
During this stage, your body repairs its tissues, and re-builds muscles and bones.
Being in this stage gives your immune system a boost.
As we age, we spend less time in this stage of sleep.
This stage of sleep is when conditions like sleepwalking and bedwetting are more likely to occur.
Related: 12 Signs of Sleep Deprivation You Need to Know
REM
REM (rapid eye movement) is the stage of sleep in which the bulk of your dreaming happens. The phase is characterized by your eyes moving rapidly under your eyelids. Although the vast majority of dreaming occurs during this phase, some amount of dreaming can occur in the other phases of sleep.
Facts about this stage:
REM lasts between 10 minutes and an hour, with the length of REM sleep lengthening as the night progresses.
It begins roughly 90 minutes after you fall asleep.
Your brain wave activity more closely resembles wakefulness.
Your breathing becomes more rapid, your heart beats faster, and your blood pressure resembles your waking blood pressure.
During REM sleep, your body experiences a kind of paralysis, so that it isn’t able to act out what is happening in your dreams.
The older you get, the less time you spend in REM sleep.
This stage of sleep is when sexual arousal is most likely to occur.
Related: Wet Dreams Are Normal. Here’s Why They Happen
Factors That Affect Your Sleep Cycle
The quality of sleep you get and the amount of time you spend in each sleep cycle depends on several variables. It’s not uncommon to have times where you don’t progress perfectly through each stage.
There are several factors that can alter your sleep cycles, such as:
Age: As we age, we tend to spend less time in deep sleep and less time in REM sleep.
Mental health: Sleep quality is impacted by mental health, including depression and anxiety. Clinical depression increases time in REM sleep, but decreases the time between falling asleep and the beginning of the first REM cycle.
Sleep disorders: Disorders such as circadian rhythm disorders, sleep apnea, REM sleep disorder, sleep walking, and narcolepsy, can impact sleep cycles.
Traumatic brain injury (TBI): Living with a TBI can have strong impacts on sleep cycles, including fewer minutes in REM sleep, increased nighttime wake-ups, and overall less time sleeping.
Medication and drugs: Alcohol, barbiturates, and benzodiazepines are known to lessen the amount of time spent in REM sleep.
Related: What Really Happens to Your Body When You Hit Snooze?
Frequently Asked Questions
What happens if you're deprived of REM sleep?
There isn’t much research about what happens when you don’t spend enough time in REM sleep. However, less REM sleep has been associated with decreased cognitive functioning and increased rates of depression.
Why do people wake up at 3 a.m.?
Waking up in the middle of the night is known as nocturnal awakening. People wake up in the middle of the night for all sorts of reasons, such as having to pee, disruptive sounds, drinking too much coffee or alcohol close to bedtime, or experiencing stress or anxiety. Nocturnal awakenings can also be caused by sleep apnea, periodic limb movement disorder, nocturnal seizures, and other sleep disorders.
What stage of sleep cycle is best to wake up to?
We usually wake up in an REM cycle, which is when we are in a lighter phase of sleep, so that the transition to wakefulness is easier. The worst phase of sleep to wake up to is NREM stage three, the deepest stage of sleep. People who wake up during this phase are usually groggy, and may experience sleep inertia, which is temporary disorientation and decline in mood after awakening from sleep.
A Quick Review
The four stages of sleep describe the four types of sleep your body cycles through during the night. These stages include three non-rapid eye movement (NREM) stages, along with one REM (rapid eye movement) stage. During a full night’s sleep, you cycle through all four stages for about 90 minutes at a time, and you do this about half a dozen times each night.
Cycling through the four stages of sleep is important for your physical and mental health. If you are having trouble sleeping, are waking up frequently, or find that you don’t wake up feeling restored, reach out to your healthcare provider. They may be able to help you pinpoint what is disrupting your sleep cycles, and offer suggestions for how to remedy this.
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