Sleep Stages: Understanding Non-Rem Phases

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. Each phase and stage of sleep include variations in muscle tone, brain wave patterns, and eye movements.

When you first fall asleep, you enter the N1 stage, the lightest stage of sleep. This usually lasts just one to seven minutes. During this stage, the body has not fully relaxed, though the body and brain activities start to slow with periods of brief movements.

The next stage is N2, which is still light sleep but deeper than N1. During this stage, your heart rate and body temperature drop, and your breathing slows. This stage accounts for about 45% of your time asleep.

The final stage is N3, also known as slow-wave sleep (SWS). This is the deepest stage of sleep and is characterised by slower brain waves. It is harder to wake someone up during this stage, and if they do wake up, they will probably experience sleep inertia, a state of confusion or mental fog that can last up to 30 minutes.

NREM stage 1: the lightest stage of sleep

NREM stage 1 is the lightest stage of sleep. It is the first stage of sleep that you enter right after you fall asleep. This stage usually lasts just one to seven minutes, making up about 5% of your total sleep time. During NREM stage 1, the body has not fully relaxed, but the body and brain activities start to slow down 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 sleep stage, but if a person is not disturbed, they can quickly move into NREM stage 2. As the night goes on, an uninterrupted sleeper may not spend much more time in NREM stage 1 as they move through further sleep cycles.

During NREM stage 1 sleep, your brain slows down, and your heartbeat, eye movements, and breathing slow with it. Your body relaxes, and your muscles may twitch. This is the stage when you are still somewhat aware of your surroundings and can be woken up easily.

As you progress into NREM stage 2, your body temperature drops, your muscles relax further, and your eye movements stop. Your breathing and heart rate become more regular. The brain also begins to produce bursts of rapid, rhythmic brain wave activity, known as sleep spindles, which are associated with memory consolidation.

NREM stage 2 is longer than NREM stage 1, typically lasting for 10 to 25 minutes during the first sleep cycle, and each successive NREM stage 2 becomes longer. Collectively, a person spends about half of their sleep time in this stage.

NREM stage 3 is the deepest stage of sleep, also known as slow-wave sleep (SWS). It is harder to wake someone up during this stage, and they will likely experience "sleep inertia," a state of confusion or "mental fog" that can last for about 30 minutes upon waking. During NREM stage 3, your muscle tone, pulse, and breathing rate decrease as your body relaxes even further. The brain activity during this stage is characterised by delta waves, which are slow brain waves with high amplitudes.

NREM sleep is crucial for restorative sleep, allowing for bodily recovery, growth, and strengthening of the immune system. As the night progresses, NREM stage 3 sleep gets shorter, and more time is spent in REM sleep.

NREM stage 2: the brain begins to produce sleep spindles

NREM stage 2 is a deeper sleep than stage 1. During this stage, the heart rate and body temperature drop, and eye movements stop. Brain waves slow down and there are noticeable pauses between short, powerful bursts of electrical activity. These bursts are known as sleep spindles, which are brief bursts of neuronal firing in the superior temporal gyri, anterior cingulate, insular cortices, and thalamus. Sleep spindles induce a calcium influx into cortical pyramidal cells, which is believed to be integral to synaptic plasticity.

Sleep spindles have been shown to be essential in memory consolidation, specifically procedural and declarative memory. Numerous studies have suggested that sleep spindles help strengthen neural connections related to recently acquired memories. They also likely help shut out external stimuli so that you do not wake up as easily. Sleep spindles have a frequency of approximately 7 to 15 Hertz and can be classified into two categories: slow or fast, representing activity in different areas of the brain.

Stage 2 NREM sleep accounts for about 45% of your time asleep (the most of any stage). You will go through multiple rounds of stage 2 NREM sleep, and usually, each one is longer than the last. After stage 2, you move into the deeper stage 3 NREM sleep or enter REM sleep.

NREM stage 3: the deepest stage of sleep

NREM stage 3 is the deepest stage of sleep, also known as slow-wave sleep (SWS) or delta sleep. During this stage, the body relaxes even further, with muscle tone, pulse, and breathing rate decreasing. Brain activity during this period is characterised by delta waves, which are slower brain waves that indicate healing and restorative sleep.

NREM stage 3 is the most challenging stage 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. As people age, they spend less time in this slow, delta-wave sleep and more time in the lighter stages of sleep.

NREM stage 3 is critical for restorative sleep, allowing the body to repair and regrow tissues, build bone and muscle, and strengthen the immune system. It is also believed to contribute to insightful thinking, creativity, and memory consolidation.

During the early sleep cycles, NREM stage 3 typically lasts for 20 to 40 minutes. As the night progresses, these stages become shorter, and more time is spent in REM sleep.

Children get the most deep sleep, while teenagers get less, and the amount of deep sleep adults get continues to decrease with age.

REM sleep: the dreaming stage

REM sleep, or rapid eye movement sleep, is the fourth stage of the sleep cycle and is characterised by several physiological changes. These include muscle relaxation, eye movement, faster respiration, and increased brain activity. This stage usually occurs around 90 minutes after falling asleep and typically happens every 90 to 120 minutes. People spend about 25% of their total sleep time in REM sleep.

During REM sleep, the body experiences atonia, a 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. Brain activity during REM sleep is similar to that of a waking person, and it is in this stage that most dreams occur.

REM sleep is believed to be essential for cognitive functions such as memory, learning, and creativity. It may also play a role in emotional processing and the development of the central nervous system. Deprivation of REM sleep has been linked to various health concerns, including increased appetite and weight, and memory consolidation issues.

The transition from NREM sleep to REM sleep is marked by electrical bursts called ponto-geniculo-occipital waves (PGO waves) originating in the brain stem. The brain stem and the limbic and paralimbic systems show increased activation during REM sleep. The superior frontal gyrus, medial frontal areas, intraparietal sulcus, and superior parietal cortex exhibit similar activity levels during REM sleep and wakefulness.

REM sleep is initiated by the secretion of acetylcholine and the inhibited production of serotonin and other monoamines. It is characterised by the suspension of homeostasis, rapid eye movements, and increased brain activity.

Sleep disorders: conditions that can negatively impact sleep quality

Sleep disorders are conditions that can negatively impact sleep quality, quantity, and timing. There are over 80 types of sleep disorders, and they can affect both mental and physical health. Here are some common sleep disorders that can disrupt sleep quality:

• Insomnia: Insomnia is a common sleep disorder characterised by difficulty falling or staying asleep. It can lead to fatigue, irritability, and impaired daytime functioning.
• Obstructive Sleep Apnea: This disorder is characterised by snoring and moments of interrupted breathing during sleep. It disrupts sleep quality and can lead to excessive daytime drowsiness.
• Restless Legs Syndrome: People with restless legs syndrome experience an urge to move their legs when trying to rest, making it challenging to fall asleep or stay asleep.
• Narcolepsy: Narcolepsy is a sleep disorder where individuals struggle to regulate their sleep patterns, often falling into REM sleep prematurely. This results in frequent daytime sleepiness and brief episodes of muscle weakness.
• Shift Work Sleep Disorder: This disorder affects those who work non-traditional hours, making it difficult to fall asleep, stay asleep, and wake up at the desired times.
• Delayed Sleep Phase Syndrome: People with this disorder fall asleep much later than their desired bedtime and have trouble waking up for school or work.
• REM Sleep Behaviour Disorder: This disorder is characterised by acting out dreams through vocalisations and sudden limb movements during REM sleep.

These are just a few examples of sleep disorders that can negatively impact sleep quality. Sleep is essential for our overall health and well-being, and when disrupted, it can have far-reaching consequences on our daily lives and long-term health.

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