Consistently getting good quality sleep is important for your health. But what exactly happens when you sleep? And how does it help your body?
As you sleep, you cycle through four different sleep stages. Each stage brings changes to your brain waves, muscles and eye movements. And they each do something different to help your body.
This article will describe everything you need to know about sleep cycles—from when they occur to what they do.
What are sleep cycles?
A sleep cycle is made up of four stages:
- 1 rapid eye movement (REM) stage
- 3 non-REM stages (NREM stages 1, 2 and 3)
Typically, people go through 4-6 sleep cycles in a night. The length of each stage can vary. An entire sleep cycle lasts around 90 minutes.
Sleep cycles are shortest at the beginning of sleep (70-100 minutes). They lengthen as the night goes on (90-120 minutes).
Sleep cycles are regulated by the circadian rhythm. This is the body’s clock that helps tell the body when to initiate changes in brain function and the release of hormones.
A typical sleep cycle looks like this:
Awake > Stage 1 > Stage 2 > Stage 3 > Stage 2 > REM > Awake
What happens in each sleep stage?
Stage 1 (5% of sleep)
The first stage of sleep is a quick transition from relaxed wakefulness to being asleep. This is when:
- You drift to sleep
- Your body begins to relax
- Your muscles may still move
- It is easy to wake up from this sleep stage, and you might not even realize you were asleep
This stage only happens when you first fall asleep and lasts 1-5 minutes.
Stage 2 (45% of sleep)
During stage 2, your body grows even more relaxed as you move into deeper sleep. This is when:
- Your heart rate and breathing slow down
- Your body temperature drops
- Your muscles relax
- Your eye movement stops
- Memories become consolidated
- Teeth grinding can occur
The greatest amount of time spent sleeping is in stage 2.
Stage 3 (25% of sleep)
Stage 3 of sleep is the deepest sleep stage. This stage is when:
- Your body is most relaxed
- You experience delta brain waves (“slow-wave”)
- Sleep is the most restorative, as your body heals and repairs
- It’s hard to wake up from this stage
- If you’re woken up, grogginess can last for 30-60 minutes
- Sleepwalking, night terrors or bedwetting might occur for some people
During this restorative stage of sleep, the body grows and repairs tissues, strengthens the immune system and builds bone and muscle.
Most stage 3 sleep happens during the first half of the night. This stage gets shorter as the night goes on.
With age, people spend less time in stage 3 and more time in stage 2.
REM (25% of sleep)
As the name describes, REM sleep is characterized by rapid eye movement. During REM sleep:
- You dream
- Your brain is very active, and brain waves are similar to being awake (beta waves)
- Your pulse and blood pressure increase and vary
- You get the deepest sleep, but it is not considered the most restful
The first REM stage happens around 90 minutes after falling asleep and lasts for around 10 minutes. With each sleep cycle, the REM stage grows longer. During the last sleep cycle, REM may last up to an hour.
| Sleep stage | Also called | Length | Characteristics/purpose |
| Stage 1 |
N1 Light sleep |
1-5 minutes |
Falling asleep stage Lightest stage of sleep |
| Stage 2 |
N2 Deeper sleep |
25 minutes (first cycle), then longer throughout the night
|
Heart rate and breathing slows Body temperature drops Muscles relax Memory consolidation Teeth grinding can happen here |
| Stage 3 |
N3 Deepest sleep |
20-40 minutes, getting shorter throughout the night |
Deepest sleep; hard to wake from; slow waves (delta waves) Restorative sleep Sleepwalking occurs during this stage Less time spent in this stage with age |
| REM | 10-60 minutes with cycles getting longer throughout the night | Dreaming/not restful; beta waves (similar to wakeful brain waves); people often wake up in the morning during this stage |
Factors that affect sleep cycles
Certain health conditions can interrupt sleep cycles, causing more health problems. Here are some things that can disrupt sleep:
Sleep apnea
With this condition, people have trouble breathing during deeper sleep states. This means less time spent in stage 3 and REM sleep, which can cause daytime drowsiness and irritability. It can also lead to more serious health complications, including metabolic syndrome.
Insomnia
Ten percent of people experience insomnia. This is where people have a hard time either falling or staying asleep. It can cause nighttime distress and make it harder to function during the day.
Narcolepsy
With narcolepsy, people skip the initial sleep stages and directly enter REM sleep, even during short naps. People with narcolepsy may be tired, or fall asleep briefly, during the day. They also experience a type of muscle weakness called cataplexy.
Aging
It is common for older adults to experience some disruptions to sleep, including difficulty falling and staying asleep.
Alcohol and sleeping pills
Alcohol and sleeping pills are linked with less stage 3 and REM sleep. They also make it harder to wake up from these sleep stages.
REM sleep disorder
During REM sleep, the body is usually unable to move. With this type of sleep disorder, people will talk or move, particularly when experiencing unpleasant dreams.
If you wake up without feeling rested, or feel tired during the day, talk to your doctor to rule out health conditions that may be interfering with your sleep. Plus, you can set yourself up for sleep success by taking steps to get better quality sleep. Learn how.