Gina Johnson
Sleep is an essential part of life, and humans spend about 25% of their lives sleeping. Sleep is divided into five stages: wake, N1, N2, N3, and REM. Each stage of sleep has distinct characteristics, and understanding them can help explain how sleep disorders, such as insomnia and sleep apnea, can impact sleep quality and overall health. This topic will be further explored by discussing the various sleep stages and the roles they play in ensuring we wake up feeling refreshed.
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What You'll Learn
Sleep stages
Sleep is divided into two distinct 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. Each phase and stage of sleep includes variations in muscle tone, brain wave patterns, and eye movements.
The first stage of sleep is N1, which is when a person first falls asleep. This stage normally lasts just one to seven minutes. During N1 sleep, the body has not fully relaxed, though the body and brain activities start to slow with periods of brief movements. It is easy to wake someone up during this stage, but if a person is not disturbed, they can move quickly into stage 2.
Stage 2, or N2, is a lighter stage of sleep from which you can still be awoken easily. During this stage, the body enters a more subdued state including a drop in temperature, relaxed muscles, and slowed breathing and heart rate. At the same time, brain waves show a new pattern and eye movement stops. On the whole, brain activity slows, but there are short bursts of activity that help resist being woken up by external stimuli.
The final stage of non-REM sleep, N3, is the deepest sleep stage. During this stage, muscle tone, pulse, and breathing rate decrease even further as the body relaxes. The brain activity during this period 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). It is harder to wake someone up during this stage. 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, which is typically when you dream. During this stage, your eyes move behind your eyelids, and your brain activity is similar to brain activity while you are awake. Your muscles usually become limp to prevent you from acting out your dreams. You usually have more REM sleep later in the night, but you do not have as much REM sleep in colder temperatures. Each of your later REM stages gets longer, and the final one may last up to an hour. REM sleep is important because it stimulates the areas of your brain that help with learning and memory. During this stage, your brain repairs itself and processes emotional experiences. It also transfers short-term memories into long-term memories.
Over the course of the night, a person goes through four to six sleep cycles, each composed of the four individual stages described above. A typical night's sleep consists of four to five sleep cycles, but this can vary depending on the individual. The progression of sleep stages in a typical night is as follows: N1, N2, N3, N2, REM.
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Sleep disorders
Other common sleep disorders include sleep apnea, restless leg syndrome, hypersomnia, circadian rhythm disorders, and parasomnia.
Sleep apnea is a breathing disorder characterised by temporary pauses in breathing during sleep, lasting 10 seconds or longer. It includes central sleep apnea, caused by the brain's failure to signal respiratory muscles, and obstructive sleep apnea, resulting from a partial or complete blockage of the upper airway. Sleep apnea leads to reduced time in deep sleep and REM sleep, causing excessive daytime drowsiness. Treatment options include CPAP machines and lifestyle changes.
Restless leg syndrome (RLS) is another prevalent sleep disorder, characterised by a tingling or prickly sensation in the legs, accompanied by an irresistible urge to move them. Hypersomnia, on the other hand, is marked by an inability to stay awake during the day, encompassing narcolepsy, which induces extreme daytime sleepiness. Circadian rhythm disorders disrupt the sleep-wake cycle, making it challenging for individuals to sleep and wake at appropriate times.
Parasomnia refers to unusual behaviours during sleep transitions, such as walking, talking, or eating. This can include REM sleep behaviour disorder, where individuals may act out their dreams due to disturbances in the usual muscle paralysis during REM sleep. Understanding these sleep disorders and their unique characteristics is crucial for developing effective treatments and improving overall sleep health.
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Sleep cycles
Sleep is not a uniform activity. Instead, a typical night's sleep consists of 4 to 6 sleep cycles, each composed of four stages: wake, N1, N2, and REM. The first three are non-rapid eye movement (NREM) sleep stages, with each stage leading to progressively deeper sleep. The fourth stage, REM, is the rapid eye movement stage.
The first stage, N1, is when a person first falls asleep. This stage normally lasts just one to seven minutes. During N1 sleep, the body has not fully relaxed, though the body and brain activities start to slow with periods of brief movements. It is easy to wake someone up during this stage. If a person is not disturbed, they can move quickly into the second stage, N2.
During N2 sleep, the body enters a more subdued state, including a drop in temperature, relaxed muscles, and slowed breathing and heart rate. Brain waves show a new pattern, and eye movement stops. On the whole, brain activity slows, but there are short bursts of activity that help resist being woken up by external stimuli. N2 sleep comprises the largest percentage of total sleep time.
The third stage, N3, is the deepest sleep stage. Muscle tone, pulse, and breathing rate decrease in N3 sleep as the body relaxes even further. The brain activity during this period 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). It is harder to wake someone up if they are in this phase. 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 fourth and final stage is REM sleep, during which the eyes twitch and the brain is active. Brain activity during REM sleep is similar to brain activity during waking hours. Dreaming usually happens during REM sleep, and the muscles become limp to prevent the sleeper from acting out their dreams. Each successive REM stage gets longer, and the final one may last up to an hour. REM sleep is important because it stimulates the areas of the brain that help with learning and memory. During this stage, the brain repairs itself, processes emotional experiences, and transfers short-term memories into long-term memories.
The duration spent in each sleep stage changes as individuals age, reflecting a decline in the overall biological necessity for sleep over time. Newborns, for example, spend more time in REM sleep, while older people may not have any slow-wave sleep at all.
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Sleep duration
The duration spent in each sleep stage also varies as individuals age. Newborns, for example, have irregular sleeping and waking patterns, with sleep distributed evenly across day and night for the first few weeks of life. They spend more time in REM sleep, which is important for brain development and growth. As people mature, the patterns and types of sleep change. The amount of slow-wave sleep, which is characteristic of deep sleep, peaks in early childhood and then decreases with age.
During a typical night's sleep, an individual will progress through the five stages of sleep: wake, N1, N2, N3, and REM. N1 to N3 are considered non-rapid eye movement (NREM) sleep, with each stage leading to progressively deeper sleep. Approximately 75% of sleep is spent in the NREM stages, with the majority spent in the N2 stage.
The first stage, N1, is when an individual first falls asleep. This stage normally lasts from one to seven minutes, and the body and brain activities start to slow, although brief movements may still occur. It is easy to wake someone during this stage, but if uninterrupted, they can quickly transition to the next stage, N2.
N2 is a lighter stage of sleep, comprising the largest percentage of total sleep time. Muscle relaxation, decreased heart rate and breathing, and eye movement cessation characterise this stage. While it is still possible to wake someone in this stage, external stimuli become less likely to disturb the sleeper.
N3, also known as delta sleep or slow-wave sleep, is the deepest stage of sleep. It is harder to wake someone during this phase as the body undergoes various health-promoting functions, including repair, recovery, and immune system enhancement. This stage is critical for feeling rested upon waking up.
Following N3 is the REM stage, during which individuals typically experience dreams. The eyes move rapidly behind closed eyelids, and the brain exhibits activity similar to wakefulness. The duration of REM sleep increases with each cycle, with the final cycle lasting up to an hour. REM sleep is important for learning, memory, and emotional processing.
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Sleep quality
Sleep is a very important function for our body to restore and repair. A typical night's sleep consists of 4 to 6 sleep cycles, with each cycle composed of four individual stages. These are wake, N1, N2, N3, and REM.
N1 is the first stage of sleep, where the body has not fully relaxed, and brain and body activities start to slow, with periods of brief movements. It is easy to wake someone up during this stage. N2 is a light stage of sleep, where the body enters a more subdued state, including a drop in temperature, relaxed muscles, and slowed breathing and heart rate. Brain activity slows, but there are short bursts of activity that help resist being woken up by external stimuli.
N3 is the final stage of non-REM sleep and is the deepest sleep stage. During this stage, the body performs various health-promoting functions, including repairing injuries and reinforcing the immune system. It is harder to wake someone up during this stage.
The REM stage is when most dreams occur, and brain activity is similar to brain activity during waking hours. The eyes move behind the eyelids, and the muscles become limp to prevent the sleeper from acting out their dreams. Each later REM stage gets longer, with the final one lasting up to an hour.
Sleep disorders can negatively impact sleep quality and cause other health problems. Examples of sleep disorders include insomnia, sleep apnea, and narcolepsy. Proper sleep hygiene, such as maintaining a consistent sleep schedule and avoiding bright lights and electronics before bed, can help improve sleep quality.
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Frequently asked questions
There are four stages of sleep: three non-rapid eye movement (NREM) sleep stages (N1, N2, and N3) and one rapid eye movement (REM) sleep stage.
REM sleep is the stage of sleep where most dreams occur. It is characterised by rapid eye movement, increased brain activity, and muscle limpness.
NREM sleep is the period of sleep where the body and brain activities slow down. It is composed of three stages, with N3 being the deepest sleep stage.
A typical night's sleep consists of four to six sleep cycles, each lasting around 90 minutes.
If you don't get enough REM sleep, you may experience symptoms such as difficulty concentrating, irritability, and impaired memory consolidation.
