Stages of Sleep: The Definitive Guide (2024)

stages of sleep

We all know that sleep plays an important role in maintaining a healthy lifestyle. It’s the medicine we require each day to maintain a well-functioning mind and body.

We spend around a third of our lives sleeping, but what actually happens when we sleep?

Prior to the 1950s, we didn’t really know much about sleep — it was thought of as a passive activity that helped restore our bodies overnight. But, in 1953, scientists discovered that sleep consists of different stages. During an average night’s sleep, our bodies go through several sleep stages, and each is incredibly important for our overall health, memory and development.

While we are catching up on some much-needed shut-eye, our brains and bodies are wide awake repairing themselves, logging memories and re-energizing for the day ahead. Studies have also found that sleep helps restore the brain by flushing out toxins that build up while we’re awake.

The amount of sleep we get can massively affect our mood, health and performance in day to day life. Lack of sleep is related to a number of physical and mental health conditions, so it’s vital we get the recommended amount of sleep each night.

In this article, we’re going to delve into the land of nod and explore the different stages of a sleep cycle. Read on to find out what really happens while we sleep.

How Do We Monitor Sleep Cycles?

Our sleep patterns are measured through a study called polysomnography. A polysomnography monitors our brains and bodies while we sleep, paying particular attention to the following:

  • Brain waves
  • Eye movements
  • Heart rate
  • Oxygen levels
  • Breathing pattern
  • Limb movement

The main test used to monitor sleep cycles is an EEG. During an EEG (electroencephalography), small sensors are placed on the scalp to measure brain activity. By monitoring and recording our brain waves as we sleep, an EEG can determine what stage of sleep we are in, whether we are moving through the stages correctly, and help diagnose a variety of sleep disorders.

Other tests performed during a polysomnography include an electrocardiogram (ECG), electrooculography (EOG), electromyography (EMG) and oxygen saturation. Each of these tests measures a different aspect of the body while we are asleep.


The Stages of Sleep

So, what are the stages of sleep, how long does each one last, and what’s the importance of each stage?

Altogether, there are four stages of sleep, and these fall into two categories: non-REM and REM. Our brain cycles through both types of sleep a number of times throughout the night.

Let’s take a look at the sleep stages in a bit more detail below:

Non-REM Sleep

We spend the majority of our night in non-REM sleep. This type of sleep is thought to be important for learning, memory and rest.

Stage 1

Stage 1 non-REM sleep is the transition between being awake and falling asleep. During this stage of light sleep, which lasts up to seven minutes, our muscles begin to relax (which can result in our body twitching), and our heartbeat and breathing slow down. We are easily awakened during this stage.

Stage 2

Stage 2 non-REM sleep is a deeper phase of sleep than stage 1, but it is still relatively light, and we are still easily awakened. This stage usually lasts 10-25 minutes, but actually makes up 50% of our overall sleep time. Stage 2 is thought to be the ‘memory consolidation’ phase when our brain stores long term memories. It’s the final stage before we go into a deep sleep, so our muscles, heartbeat and breathing relax even more, our body temperature drops and eye movements stop.

Stage 3 (Slow Wave Sleep)

Stage 3 non-REM sleep (previously referred to as stage 3 and 4) is a deep sleep that lasts around 20-40 minutes. The body becomes less responsive to the outside world during this stage, and it’s harder for us to be awoken. During the first half of our sleep cycle, stage 3 happens in longer intervals. It’s an important stage of sleep and contributes towards feeling refreshed and re-energized when we wake up.

REM Sleep

After stage 3, we enter REM, or Rapid Eye Movement sleep — this occurs around 90 minutes after initially falling asleep. REM sleep is when we experience most dreaming. However, our muscles become temporarily paralyzed to stop us from reenacting our dreams. The first REM cycle we go through may only last up to five minutes, but the length of time we spend in REM sleep increases throughout the night, which is why it’s common to be woken up dreaming in the morning.

During REM sleep, our eyes flicker beneath our eyelids (hence the term ‘rapid eye movement’), breathing becomes faster and our heart rate and blood pressure increase. It’s also referred to as ‘paradoxical sleep’ because our brain activity is similar to when we are awake, yet our body becomes immobile.

The amount of time we spend in REM sleep decreases with age. Babies are thought to spend 50% of their sleep time in REM, while adults spend 20-25% of sleep time in REM.

What Does a Full Sleep Cycle Look Like?

During an average night’s sleep, we move through all four stages of sleep several times, alternating between non-REM and REM sleep. As we go through stages 1 to 3, our brain waves begin to slow, yet when we get to REM sleep, our brain wave activity resembles that of wakefulness.

The average length of our initial full sleep cycle (from non-REM to REM) is around 70-100 minutes. However, this changes as the night progresses, with later cycles averaging 90-120 minutes.

Studies have found that our initial sleep cycle moves sequentially through stages 1, 2 and 3 then reverts back to stage 2 before entering REM sleep. After the initial sleep cycle we go from REM sleep straight to stage 2. We continue the same sequence roughly five times throughout the night, with the length of REM sleep increasing as morning draws nearer. As mentioned previously, stage 2 makes up the majority of our sleep cycle, with stage 3 sometimes disappearing altogether.

how to get to sleep

Sleep Disorders and Sleep Stages

Sleep disorders can cause irregular and interrupted sleep cycles and prevent individuals from getting quality sleep. Let’s take a look at some of the ways sleep disorders can affect our sleep and at what stage in our sleep cycle they occur.


Rather than entering REM sleep after 90 minutes of non-REM sleep, individuals with narcolepsy enter REM sleep very rapidly — often falling directly into REM, or within 15 minutes of falling asleep. People with narcolepsy can also experience muscle paralysis and dream activity (characteristics of REM sleep) whilst awake.

Sleep Apnea

Sleep apnea can occur during all stages of sleep, but it’s often worse during REM sleep when our muscles relax. In fact, some people only experience sleep apnea during REM sleep. One study found that individuals with sleep apnea experienced a reduced amount of stage 3 and REM sleep. The study also concluded that symptoms like snoring, excessive fatigue, sleepiness, falling asleep during the day, trouble paying attention, and insomnia were linked to less stage 3 sleep.

Non-REM Parasomnias

Non-REM parasomnias include disorders like sleepwalking, night terrors and bedwetting. They are most likely to occur during our deepest sleep, which is stage 3.

How Much Sleep Do We Need?

While the amount of sleep we need varies from person to person, and much of it comes down to genes, there is certainly a ballpark figure to aim for to ensure you’re getting enough sleep each night. The majority of adults will feel refreshed and ready for the day after seven to nine hours sleep, but over a third of US adults report getting less than this at night.

According to the National Sleep Foundation, the recommended sleep time for babies is 14-17 hours, while school-aged children require 9-11 hours and teens should aim for 8-10 hours a night. The reason we require more sleep when we are younger is because our bodies are still developing, and sleep aids development.

As we’ve learned throughout this article, alot happens in our bodies while we’re sleeping. Each night, our brain and body work hard to keep us ticking along, so ensuring we get enough quality sleep is vital for our overall health.

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Getting a good night’s sleep is often easier said than done, though, with many of us regularly struggling to get to sleep or stay asleep. Whether it be due to lifestyle factors or poor sleep hygiene, lack of sleep, and interrupted sleep, can have a detrimental effect on our daily life and health.

If you’re struggling to nod off at night, why not take a look at some of our tips on getting a good night’s sleep?


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