Colin Snow
Sleep is divided into two phases: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, from N1 (light sleep) to N3 (deep sleep). N3, also known as slow-wave sleep, is the deepest and most restorative stage of sleep, where the body performs various health-promoting functions. During this stage, the predominant EEG activity consists of low-frequency, high-amplitude fluctuations called delta waves. It is difficult to wake someone from slow-wave sleep, and if they do wake up, they may experience sleep inertia, a state of confusion or mental fog.
| Characteristics | Values |
|---|---|
| Stage name | N3, deep sleep, slow-wave sleep |
| Sleep cycle | First occurs 90 minutes after falling asleep |
| Duration | 10 minutes in the first cycle |
| Difficulty to wake up | Very hard to wake someone up from this stage |
| Brain waves | Slow but strong |
| Body functions | Body repairs injuries and reinforces the immune system |
| Sleep quality | Most restorative sleep |
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What You'll Learn
Dreaming occurs during REM sleep
REM sleep is best known for its association with vivid dreaming. Dreams that occur during REM sleep are often intense and vivid, and you are more likely to remember them. This is because brain activity during REM sleep is similar to brain activity when you are awake. In fact, brain activity can sometimes be even more heightened during REM sleep than when you are awake.
During REM sleep, the brain functions at levels that are similar to when you are awake. This is why you are more likely to remember dreams that occur during this stage of sleep. The content of dreams can have a noticeable impact on your day, and how your day goes can also affect your dreams at night. Dreaming is thought to be one way that the brain processes emotions and memories. Dreams are also thought to help the mind recover from distressing experiences by suppressing them.
REM sleep usually happens 90 minutes after falling asleep. The first period of REM sleep typically lasts 10 minutes, with each subsequent REM stage getting longer. The final REM stage may last up to an hour. REM sleep makes up about 25% of total sleep time, with most adults needing about two hours of REM sleep each night.
While dreaming occurs mostly during REM sleep, it can also happen during non-REM sleep. Evidence suggests that dreams that occur during non-REM sleep are "less remarkable in quantity, vividness, and emotion" than dreams that occur during REM sleep.
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Brain activity increases during REM sleep
Sleep is divided into four stages, including one for rapid eye movement (REM) sleep and three that form non-rapid eye movement (NREM) sleep. During REM sleep, brain activity increases and is similar to the brain activity observed during wakefulness. The brain waves during REM sleep are fast, low amplitude, and desynchronized, resembling the pattern seen during wakefulness. This is in contrast to the slow delta waves typically observed during NREM deep sleep.
REM sleep constitutes a distinct "third state" of consciousness, where brain activity levels are comparable to those during wakefulness, yet conscious awareness is significantly altered. During this stage, the brain exhibits a more dynamic interaction between unimodal sensorimotor areas and higher-order brain systems. The thalamus, which usually quiets down during sleep, becomes active during REM sleep. It sends the cortex images, sounds, and other sensations that fill our dreams. The amygdala, involved in processing emotions, also shows increased activity during REM sleep.
The activation-synthesis hypothesis proposed by Robert McCarley and Allan Hobson suggests that REM sleep is regulated by "REM-on" and "REM-off" neurons in the brain stem. REM-on neurons are primarily cholinergic, while REM-off neurons activate serotonin and noradrenaline, which suppress the REM-on neurons. This cyclical mechanism controls the transition between REM and non-REM sleep.
The duration and quality of sleep can vary with age, and certain disorders or conditions can disrupt the sleep cycle. For example, individuals with narcolepsy may enter REM sleep directly, experiencing dreams during short naps. Additionally, factors such as age, recent sleep patterns, and alcohol consumption can influence the sleep cycle.
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The body becomes temporarily paralysed during REM sleep
Sleep is divided into two types: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, numbered N1 to N3.
During REM sleep, the body becomes temporarily paralysed. This is caused by the brain chemicals gamma-aminobutyric acid (GABA) and glycine, which switch off the specialised cells in the brain that allow muscles to be active. This paralysis prevents the body from acting out dreams, keeping people still even as their brains are active. It also prevents injury, as people remain still during sleep.
The paralysis of the body during REM sleep can result in sleep paralysis, where an individual regains awareness while entering or exiting the sleep phase, but their body has not yet moved out of the sleep paralysis phase. This can be a frightening experience, and individuals may feel scared, anxious, tired, or confused during and after an episode. Sleep paralysis can happen to anyone at any age, but it is more common among people with irregular sleep schedules.
The understanding of the mechanism behind sleep paralysis has pointed researchers towards developing treatments for sleep-related motor disorders, such as REM sleep disorder. This disorder causes individuals to act out their dreams, which can result in injury to themselves or those around them.
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Slow brain waves during N3 sleep
Sleep is divided into two phases: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, numbered N1 to N3. The first stage of NREM sleep is the lightest, with the sleeper's body not fully relaxed, and the brain and body activities starting to slow down. N3 sleep, also known as slow-wave sleep (SWS), is the deepest and most restorative stage of sleep. During this stage, the brain produces slow, strong, and high-voltage delta waves interspersed with sharp waves. These sharp waves originate in the hippocampus of the brain and are associated with memory processing.
The slow brain waves of N3 sleep are characterised by lower frequencies and higher amplitudes, with peak-to-peak amplitudes greater than 75 microvolts. This stage is the most difficult to wake someone up from. If they do wake up, they will likely experience sleep inertia, a state of confusion or mental fog that can last up to 30 minutes. N3 sleep is essential for the healing of the body and the formation of memories. It is also when the brain clears itself of toxins and harmful substances.
The duration of N3 sleep episodes varies with age. Children and young adults will have longer episodes of N3 sleep than older adults. Additionally, adolescents tend to have longer REM sleep latencies than children, resulting in more time spent in N3 sleep. As people age, they spend less time in N3 sleep and more time in the lighter N2 sleep stage.
The amount of N3 sleep a person gets can be affected by various factors. For example, individuals recovering from sleep deprivation may experience an unusually high amount of N3 sleep. In contrast, the use of medications such as benzodiazepines or tri-cyclic antidepressants may reduce the duration of N3 sleep. Sleep disorders such as insomnia and sleep apnea can also impact the amount of N3 sleep a person gets.
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N3 sleep is the deepest sleep stage
Sleep is divided into two main phases: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, N1 to N3, with each stage leading to progressively deeper sleep.
N3 sleep, also known as slow-wave sleep (SWS) or deep sleep, is the third stage of sleep and the deepest of the four sleep stages. During this stage, muscle tone, pulse, and breathing rate decrease, and brain activity has an identifiable pattern of delta waves. This is the most difficult stage to wake someone from. If they do wake up, they will likely experience sleep inertia, a state of confusion or "mental fog" that can last about 30 minutes.
N3 sleep is critical to restorative sleep, allowing the body to recover and repair injuries, and possibly boosting the immune system. It may also contribute to insightful thinking and creativity. During the early sleep cycles, N3 stages commonly last for 20 to 40 minutes. As the night progresses, N3 stages get shorter, and more time is spent in REM sleep.
The amount of time spent in each sleep stage changes as people age, with older people spending less time in N3 sleep and more time in N2 sleep. Children tend to spend more time in N3 sleep than adolescents, who experience a decline in slow-wave sleep due to pubertal and hormonal changes.
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Frequently asked questions
Stage IV sleep is the deepest level of sleep, also known as slow-wave sleep. It is the most difficult stage to wake someone up from.
During this stage, the body performs various health-promoting functions. The body repairs injuries, reinforces the immune system, and grows.
The entire sequence from drowsiness to deep stage IV sleep usually takes about an hour. Each sleep cycle, which includes stage IV sleep, lasts around 90 minutes.
On average, people cycle through all stages of sleep about 4 to 6 times each night.
