Gina Johnson
Sleep is divided into two phases: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into four stages, with each stage leading to progressively deeper sleep. The first stage, N1, is the transition from wakefulness to sleep and usually lasts a few minutes. The second stage, N2, is a period of light sleep before entering deeper sleep and lasts around 10 to 25 minutes. The third stage, N3, is the deepest period of sleep, during which the body performs various health-promoting functions. The REM stage is when most dreams occur, and the eyes move behind closed eyelids. A typical night's sleep consists of four to five sleep cycles, each lasting around 90 to 110 minutes.
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What You'll Learn
Non-rapid eye movement (NREM) sleep: the transition from wakefulness to sleep
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 to N3, which lead to progressively deeper sleep.
Stage N1 is the transition from wakefulness to sleep. It generally lasts only a few minutes and is the lightest stage of sleep. If you’re awoken from N1, you usually don’t realise that you were asleep. This stage is marked by a slowing of your heartbeat, breathing, and eye movements, as well as the relaxation of your muscles.
Stage N2 is a period of light sleep before you enter deeper sleep. It lasts roughly 10 to 25 minutes during the first sleep cycle, and each N2 stage can become longer during the night. This stage is characterised by a further slowing of the heartbeat and breathing, and the brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles. Sleep spindles are thought to be the brain organising memories and information from when you were awake. Stage N2 accounts for about 45% of your time asleep, and you will usually go through multiple rounds of it, with each round longer than the last.
Stage N3 is the final stage of non-REM sleep and is the deepest period of sleep. It lasts 20 to 40 minutes, and it is much harder to wake someone up during this stage. Muscle tone, pulse, and breathing rate decrease, and brain activity has an identifiable pattern of delta waves. This stage is critical to restorative sleep, allowing for bodily recovery and growth, as well as bolstering the immune system and other key bodily processes.
On an average night, adults typically need to complete at least four or five sleep cycles, with each cycle taking roughly 90 to 110 minutes. The first cycle is often the shortest, ranging from 70 to 100 minutes, while later cycles tend to fall between 90 and 120 minutes. The first REM period is usually the shortest, and as the night progresses, REM periods get longer and deep sleep (NREM) decreases.
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N1: the lightest stage of sleep
Sleep is divided into two phases: rapid eye movement (REM) and non-rapid eye movement (NREM). The latter is further divided into three stages, N1 to N3, with N1 being the lightest stage of sleep.
Stage N1 marks the transition from wakefulness to sleep. It usually lasts only a few minutes and makes up about 5% of your sleep time. If you are awoken during this stage, you usually won't perceive that you were actually asleep. This stage is characterised by a slowing of your heartbeat, breathing, and eye movements, as well as the relaxation of your muscles.
During N1, your brain waves slow down and there are noticeable pauses between short, powerful bursts of electrical activity. These bursts are thought to be your brain organizing memories and information from your time awake.
Sleep is a body process that allows your body to rest, repair, and restore itself. It is essential for your health and well-being. The quality of your sleep and the time spent in each stage can be altered by various factors, such as age, recent sleep patterns, and even alcohol consumption.
As the night progresses, the amount of time spent in each stage of sleep changes. For example, adults tend to have more REM sleep later in the night, while newborns spend more time in REM sleep overall.
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N2: a light sleep before deeper sleep
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 to N3, with each stage leading to progressively deeper sleep.
Stage N2 is a period of light sleep before you enter deeper sleep. It is a part of the NREM sleep cycle, which accounts for approximately 75% of sleep. N2 comprises the largest percentage of total sleep time, with a person typically spending about half of their sleep time in this stage. Each N2 stage can become longer during the night, with the first sleep cycle lasting 10 to 25 minutes, and the subsequent ones increasing in duration. During this stage, the brain waves slow down and there are noticeable pauses between short, powerful bursts of electrical activity. Experts believe that these bursts are the brain organizing memories and information from when the person was awake.
Stage N2 is characterized by a further slowing of both the heartbeat and breathing, and the brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles. This stage is still light sleep, but deeper than stage N1. It is easier to wake someone up during this stage compared to the deeper stages of sleep.
The amount of time spent in each sleep stage varies with age. Newborns spend far more time in REM sleep and may enter this stage as soon as they fall asleep. As they get older, their sleep patterns change and become more similar to those of adults, who tend to spend less time in REM sleep.
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N3: the deepest sleep stage
N3, also known as deep sleep or slow-wave sleep, is the final stage of non-rapid eye movement (NREM) sleep. This is the deepest stage of sleep, during which it is harder to wake someone up. Muscle tone, pulse, and breathing rate decrease, and brain activity slows down, producing delta waves.
This stage is critical to restorative sleep, allowing for bodily recovery and growth. It may also strengthen the immune system and other key bodily processes. Even though brain activity is reduced, there is evidence that deep sleep contributes to insightful thinking. It is during this stage that the body performs various health-promoting functions.
The duration of N3 sleep varies, lasting from 10 minutes during the first cycle to 20 to 40 minutes in subsequent cycles. The amount of time spent in N3 sleep changes as the night progresses, and it can even disappear altogether in later cycles. Typically, you will spend more time in this stage earlier in the night.
The amount of slow-wave sleep also changes as people mature. For example, newborns spend more time in N3 sleep, and it peaks in early childhood before dropping sharply in the teenage years.
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REM: the stage where dreams occur
Sleep can be divided into two phases: non-rapid eye movement (NREM) and rapid eye movement (REM). The latter is the stage of sleep where dreams occur. During REM sleep, your brain activity is similar to that of someone who is awake. It is called rapid eye movement sleep because your eyes move behind your eyelids while you dream.
The REM stage of sleep is when most dreams occur. Studies have shown that subjects awakened from REM sleep recall elaborate, vivid, and emotional dreams. In contrast, subjects awakened during non-REM sleep report fewer and less vivid dreams. Theories suggest that we dream as we consolidate memories, prepare for possible future threats, reflect on unconscious mental functions, or further develop our cognitive capabilities.
The amount of time spent in REM sleep changes as the night progresses. The first REM cycle of a sleep period is typically the shortest, lasting around 10 minutes. Each cycle that follows is longer, lasting up to an hour. REM sleep makes up about 25% of your total time asleep.
Babies and newborns spend far more time in REM sleep and may enter this stage as soon as they fall asleep. As people get older, the amount of REM sleep they get decreases.
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