Sleep is essential for our mental and physical health, and getting enough sleep helps us feel and focus better and think more clearly. We spend a third of our lives sleeping, but what happens when we fall asleep? What does our brain do all this time?
In the early 1900s, our brain was thought to just switch off during sleep and stop any activity. However, this idea was challenged by the discovery of REM sleep. Even though we are not conscious of it, our brain does important work at night.
Sleep cycle, freepik.com
What are the stages of sleep in psychology?
We go through several sleep cycles during a typical night of sleep. Each cycle lasts from 90 to even 110 minutes and consists of four different stages of sleep. We usually experience 4 to 5 sleep cycles during a typical eight-hour sleep. Each stage is characterised by specific brain activity in the form of waves distinguished by an EEG (electroencephalogram) and has its own function.
We can distinguish two types of sleep stages: non-REM (NREM) and REM sleep.
Different stages of sleep
In a typical cycle, the first three stages of sleep are non-REM sleep stages. As we pass through these stages and approach the REM sleep, our sleep gets deeper and deeper.
NREM sleep is characterised by a lack of rapid eye movements and less vivid dreams. During non-REM stages, the body is relaxed but not paralysed, heart rate and breathing slow down, and the brain shows theta or delta activity.
Stage 1 (N₁)
The first stage of sleep marks the transition between sleep and being awake and usually lasts up to 10 minutes. As we enter stage one, we can feel sleepiness and drowsiness, but our sleep is relatively light. A person waking up during this stage might not even realise they fell asleep.
At this stage, neurons start to fire in a more synchronised way, and the brain's activity moves to theta waves (4-8HZ). Muscles relax, and breathing and heart rate slow down relative to the waking state.
During the first stage of sleep, a common occurrence is hypnic jerks and sudden muscle contractions, often accompanied by a sense of falling.
Changes in brain activity during sleep are measured in a sleep lab using Electroencephalography (EEG). The activity of your brain when you are awake changes between alpha and beta activity. Alpha activity (8-12Hz) is characteristic of resting periods, and Beta activity (13-30Hz) occurs during more intense focus and attentiveness periods. You can notice that brain activity during NREM sleep slows down relative to waking activity.
Stage 2 (N₂) – Light sleep
Stage two is a light, non-REM stage of sleep. No rapid eye movements occur at this stage; muscles are relaxed, body temperature drops, and the heart rate continues to slow down.
At this stage, waveforms like K complexes and sleep spindles occur. These are rapid and brief changes in brain activity.
For example, K complexes involve a sharp drop in activity followed by a greater positive peak and a negative peak, after which the activity goes back to normal. Sleep spindles are bursts of higher frequency waves.
This stage lasts around 20 minutes. However, with each consecutive sleep cycle, this stage tends to get longer and can last for up to 60 minutes. Overall it is estimated to make up even half of the entire time we are asleep.
The occurrence of K complexes has been associated with reducing our sensitivity to non-threatening stimuli (e.g. random noise) during sleep; both K complexes and spindles are associated with memory consolidation.
Stage 3 (N3) - Deep sleep
In stage three, deep sleep or slow-wave sleep occurs. Breathing and heart rate are at their lowest, and muscles are relaxed. This stage is essential for maintaining physical health, as it is associated with strengthening the immune system, cell regeneration and muscle growth.
No eye movements occur at this stage, and the brain can rest as its metabolic rate drops and activity decreases. The brain's activity initially consists of 30 to 50 percent delta waves which are slower than 4Hz. With time, the fraction of delta wave activity increases to over 50 percent. Because our brain is resting, it is hard to wake up during this stage. Waking up may leave us feeling groggy and disoriented.
Typically, deep sleep lasts for 20 to 40 minutes; however, its duration changes depending on the sleep cycle. It is typically the longest during the first cycle and gets shorter as the night progresses. Deep sleep can even be skipped completely in the last cycles as REM sleep and stage 2 sleep get longer.
All mammals require deep sleep. However, some mammals like dolphins or whales can sleep with only one hemisphere at a time. During the night, one half of their brain falls into a deep sleep while the other remains awake so they can monitor the environment for threats.
Tired man wishing to sleep, freepik.com/pch.vector
REM sleep (N4) - Deepest stage of sleep
After the non-REM stages pass, we move to the REM sleep.
REM sleep is characterised by rapid eye movements, desynchronised electrical brain activity, increased activation of the brain, paralysis of muscles and vivid dreams.
During REM sleep, brain activity becomes desynchronised. This phenomenon is also characteristic of waking brain activity. Some brain areas may create patterns of theta activity (3-10Hz), and others produce high-frequency gamma waves (40-60Hz). However, theta activity is usually the most prevalent at this stage.
The brain's metabolic activity is usually similar to or even greater than waking activity. Since the brain is quite active, you may experience greater alertness after waking up during this stage than during slow-wave sleep. REM sleep is also when we experience vivid and immersive dreams. We are also more likely to be able to recall them if we wake up during this stage.
REM sleep is sometimes referred to as paradoxical sleep because the brain's activity at this stage is more similar to waking activity rather than the activity we observe during other sleep stages.
Heart rate and breathing get more irregular, and eyes move rapidly. Except for the eyes, the body is paralysed. This phenomenon is called REM atonia and prevents us from acting out our dreams when we sleep. Body temperature can fluctuate as the mechanisms that regulate the temperature of our body function poorly during REM sleep.
REM sleep can last for 10 to 60 minutes. In the initial sleep cycles, it is usually shorter, and its duration extends with each consecutive cycle. Rem sleep is associated with learning and brain development. We observe a longer duration of the REM stage in sleep cycles during childhood and adolescence.
Dreams can also occur during different stages of sleep, other than REM sleep. However, REM sleep is characterised by vivid and immersive dreams compared to those we might experience during other stages.
REM sleep and slow-wave sleep differences
REM sleep and slow-wave sleep can sometimes be confused. REM sleep is considered the deepest stage, deeper than deep sleep. This, considering the paradoxical brain activity during this stage, might be counterintuitive. The table below compares the characteristics of REM sleep and slow-wave sleep (also known as deep sleep).
| REM sleep | Deep sleep |
| Duration increases with each sleep cycle | Duration decreases with each sleep cycle |
| Muscles are paralysed, breathing and heart rate are irregular | Muscles are relaxed, breathing and heart rate are at their lowest |
| Rapid eye movements | No eye movements |
| Desynchronised activity, characterised by theta waves (4-8Hz), increased metabolic activity | Synchronised, slow, delta waves <4Hz, low metabolic activity |
| Complex, vivid dreaming | Dreams that occur are likely to resemble your existing memories |
Important for physical health, learning and memory consolidation |
The correct sequence of sleep stages
The sleep cycle is an example of an ultradian rhythm, a rhythm that is shorter than a circadian rhythm (repeating every 24h) and occurs throughout the 24 hours. Each sleep cycle follows a similar sequence of stages; stage one is followed by stage two, followed by deep sleep and REM sleep. The sequence doesn't change. However, during the night, the duration of different stages may change, and at the end, slow-wave sleep might not occur at all.
Length of sleep stages
Based on EEG data, we can estimate the average duration of each stage of sleep during a sleep cycle.
- Stage 1 typically lasts up to 10 minutes.
- Stage 2 lasts 20 to 60 minutes.
- Stage 3 (slow-wave sleep) lasts from 20 to 40 minutes.
- Stage 4 (REM sleep) lasts 10 to 60 minutes.
With each consecutive sleep cycle, the duration of different stages changes. During the beginning cycles, slow-wave sleep and stage 2 sleep take the longest. During consecutive cycles, stage 2 sleep and REM sleep get progressively longer and longer, while the duration of the slow-wave sleep decreases.
Stages of Sleep - Key takeaways
- Each sleep cycle lasts from 90 to 110 minutes and consists of four distinct stages of sleep.
- Non-REM sleep is characterised by a lack of rapid eye movements and less vivid dreams. During non-REM stages, the body is relaxed but not paralysed, heart rate and breathing slow down, and the brain shows theta or delta activity.
- Stage 1 lasts for up to 10 minutes and marks the transition between being awake and sleeping. The activity of the brain slows down, and the body relaxes.
- Stage 2 lasts for about 20 minutes in early cycles. At this stage, K complexes and sleep spindles occur.
- Stage 3, or slow-wave sleep, is characterised by the decreased metabolic activity of the brain and slower delta waves. It lasts around 40 minutes at first, and its duration decreases in later sleep cycles. It is important for physical health (e.g. immune function) and cognition (e.g. memory consolidation).
- REM sleep is considered the deepest stage of sleep. At this stage, the metabolic activity of the brain increases, the electrical activity becomes synchronised, and we might experience vivid, immersive dreams. The body is paralysed, except for the eyes that move rapidly. REM sleep is associated with physical health and learning.
- The sequence of stages in a sleep cycle doesn't change, but the length of different stages changes depending on the sleep cycle.
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