Miya Pearson
Sleep is divided into non-rapid eye movement (NREM) and rapid eye movement (REM) phases, with the latter being where most dreams occur. Sleep stages 3, 4, and 5 are part of the NREM and REM phases, respectively. During sleep stage 3, also known as N3 or deep sleep, the body repairs injuries, strengthens the immune system, and experiences slow and strong brain waves. Formerly known as stage 4, sleep stage 5, or REM sleep, is where dreaming occurs, brain activity increases, and the body becomes temporarily paralyzed.
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What You'll Learn
Slow brain waves, muscle relaxation, and slow breathing in stage 3
Sleep can be divided into two phases: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, N1 through N3, with each stage leading to progressively deeper sleep.
Stage 3 sleep, also known as N3 or deep sleep, is the deepest and most restorative sleep stage. It is harder to wake someone up if they are in this phase. During this stage, the body performs various health-promoting functions, allowing the body to recover and grow.
Stage 3 sleep is characterised by slow brain waves, specifically delta waves. Brain activity has an identifiable pattern of these waves, which are also present during deep sleep or a coma. During this stage, muscle tone, pulse, and breathing rate decrease, and the body enters a state of relaxation. The breathing rate becomes more erratic and irregular, and the skeletal muscles are atonic and without movement, except for the muscles that control breathing.
Slow brain waves during Stage 3 sleep are important for the consolidation of memory and cognitive function. This stage of sleep plays a crucial role in helping the brain recover from daily activities and prepare for the following day. It is also during this stage that the body releases growth hormones, repairs tissues, and strengthens the immune system.
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Tissue repair and growth hormones in stage 4
Sleep is divided into two phases: non-rapid eye movement (NREM) and rapid eye movement (REM). NREM sleep is further divided into three stages, N1 through N3, while REM sleep is the final stage. During the early sleep cycles, N3 stages commonly last for 20 to 40 minutes. As you continue sleeping, these stages get shorter, and more time is spent in REM sleep.
Stage 4, also known as REM sleep, is where most dreaming occurs, brain activity increases, and the body experiences atonia, or temporary paralysis of the muscles. While the eyes and the muscles that control breathing are exempt, the eyes move rapidly behind closed eyelids, giving this stage its name.
REM sleep is believed to be essential for cognitive functions such as memory. It is also during this stage that the body repairs and regenerates tissues. The release of the human growth hormone (HGH) during REM sleep is crucial for growth and muscle development, as well as tissue regeneration and repair.
Studies have shown a link between abnormal nocturnal HGH secretion and cognitive abnormalities. In children with psychosocial short stature, for instance, there was a gross deficit of stage 4 sleep and a shorter duration of slow-wave sleep. Improvements in sleep quality and an increase in stage 4 sleep duration were observed during a growth recovery period.
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Dreaming, brain activity, and paralysis in stage 5 (REM)
Sleep can be divided into two phases: non-rapid eye movement (NREM) and rapid eye movement (REM). The first three stages are part of the first NREM phase, while the final stage is the REM phase.
Stage 5, also known as REM sleep, is a much deeper sleep than any of the three stages of non-REM sleep. It occurs 90 minutes after sleep onset and is characterised by rapid eye movements, an almost complete paralysis of the body, and a tendency to dream. Dreaming occurs due to the significant uptick in brain activity, which increases and resembles the brain's alert state. Brain waves become faster and more similar to those observed during wakefulness.
During REM sleep, the body experiences atonia, or temporary paralysis of the muscles, except for the eyes and the muscles that control breathing. The eyes move rapidly behind closed eyelids, giving this stage its name. REM sleep is believed to be essential to cognitive functions like memory.
Dreams can occur in any sleep stage, but they are less common and less intense during NREM periods. As the night progresses, REM stages get longer, especially in the second half of the night. While the first REM stage may last only a few minutes, later stages can last for around an hour. In total, REM stages make up around 25% of sleep in adults.
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Transition from wakefulness in stage 1 (N1)
Sleep is divided into two phases: rapid-eye movement (REM) and non-rapid eye movement (NREM). NREM sleep is further divided into three stages: N1, N2, and N3. Each phase and stage of sleep includes variations in muscle tone, brain wave patterns, and eye movements.
Stage 1, or N1, is the transition from wakefulness to sleep. It usually lasts one to seven minutes, and the body hasn't fully relaxed yet. However, the body and brain activities start to slow, with periods of brief movements. There are light changes in brain activity associated with falling asleep in this stage. It is easy to wake someone up during this phase of sleep, but if the person is not disturbed, they can quickly move into stage 2. As the night goes on, an uninterrupted sleeper may not spend much more time in stage 1 as they move through further sleep cycles.
Stage 2, or N2, is a lighter stage of sleep from which you can still be awoken easily. This is the stage before you enter deep sleep. The body enters a more subdued state where the body temperature drops, muscles relax, and heart rate and breathing slow. Eye movement stops, and brain activity slows, though there are short bursts of activity.
Collectively, a person typically spends about half their sleep time in N2 sleep. Stage 3, or N3, is deep sleep, and it is harder to wake someone up during this phase. Muscle tone, pulse, and breathing rate decrease, and brain activity has an identifiable pattern of what are known as delta waves. For this reason, stage 3 may also be called delta sleep or slow-wave sleep (SWS). Experts believe that this stage is critical to restorative sleep, allowing for bodily recovery and growth. It may also bolster the immune system and other key bodily processes.
The final stage of sleep is REM sleep, where most dreaming occurs, brain activity increases, and the body becomes temporarily paralyzed, except for the eyes and the muscles that control breathing. REM sleep is believed to be essential to cognitive functions like memory.
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Light sleep, easily awakened in stage 2 (N2)
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, with each stage leading to progressively deeper sleep.
Stage 2 sleep, or N2, is a lighter stage of sleep where the body starts to relax more deeply. It is still easy to wake someone up during this phase of sleep. However, if the person is not disturbed, they can quickly move into stage 2. During this stage, the body temperature drops, muscles relax, and heart and breathing rates slow. Eye movement stops and brain activity slows, although there are short bursts of brain activity that help the person resist being woken up by external stimuli. Bruxism, or teeth grinding, also occurs during this stage.
N2 sleep typically lasts for 10 to 25 minutes during the first sleep cycle, and each N2 stage can become longer during the night. A typical night's sleep consists of 4 to 5 sleep cycles, with the progression of sleep stages in the following order: N1, N2, N3, N2, REM. A complete sleep cycle takes roughly 90 to 110 minutes. The first REM period is short, and as the night progresses, longer periods of REM sleep and decreased time in deep sleep (NREM) occur.
Collectively, a person typically spends about half of their sleep time in N2 sleep. As people age, they tend to spend less time in deep sleep (N3) and more time in stage N2 sleep. Although this stage has a high arousal threshold, if someone is awoken during this stage, they will experience a transient phase of mental fogginess, known as sleep inertia.
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Frequently asked questions
Sleep stage 3, also known as N3 or deep sleep, is the deepest and most restorative sleep stage. During this stage, muscle tone, pulse, and breathing rate decrease, and brain activity shows a pattern of delta waves. It is critical for restorative sleep, allowing the body to recover, repair injuries, and boost the immune system.
Sleep stage 4, also known as REM (rapid eye movement) sleep, is where most dreaming occurs. Brain activity increases and resembles the brain activity of a waking person. The body also experiences atonia, or temporary paralysis of the muscles, except for the eyes and breathing muscles.
There are four sleep stages, including one REM stage and three NREM (non-rapid eye movement) stages. However, some sources divide sleep into five stages, with an additional "wake" stage at the beginning.
The duration of each sleep stage varies throughout the night. On average, a complete sleep cycle, which includes all four stages, lasts about 90 to 120 minutes. The first sleep cycle is often the shortest, ranging from 70 to 100 minutes.
