Colin Snow
Sleep is an essential bodily function that allows the body to rest, repair, and restore itself. The human body cycles through two phases of sleep: rapid-eye movement (REM) and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, from light sleep in stage one to deep sleep in stage three. During REM sleep, the final stage, the eyes twitch, the brain is active, and dreaming usually occurs. The amount of time spent in each sleep stage can vary depending on age, health status, and other factors. Getting quality sleep is crucial for maintaining overall health and well-being.
| Characteristics | Values |
|---|---|
| Number of sleep phases | 2 |
| Names of sleep phases | REM (rapid-eye movement) sleep and non-REM sleep |
| Number of sleep stages | 4 or 5 |
| First sleep phase | non-REM sleep |
| Number of non-REM sleep stages | 3 or 4 |
| First stage of sleep | N1 |
| Characteristics of N1 sleep | Lightest stage of sleep, body hasn't fully relaxed, body and brain activities start to slow with periods of brief movements, light changes in brain activity |
| Second stage of sleep | N2 |
| Characteristics of N2 sleep | Body temperature drops, muscles relax, heart and breathing rate slow |
| Third stage of sleep | N3 or deep sleep |
| Characteristics of N3 sleep | Deepest and most restorative sleep, allows the body to recover and grow |
| Fourth stage of sleep | REM sleep |
| Characteristics of REM sleep | Most dreaming occurs, brain activity increases, body becomes temporarily paralyzed, eyes move rapidly |
| Sleep cycle duration | 90 to 120 minutes |
| Number of sleep cycles per night | 4 or 5 |
| Sleep cycle regulation | Circadian rhythm, driven by the suprachiasmatic nucleus (SCN) of the hypothalamus |
| Sleep duration recommendation | At least 7 hours of sleep in every 24 hours for adults |
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What You'll Learn
Non-rapid eye movement (NREM) sleep
Stage N1 is the lightest stage of sleep and occurs when a person first falls asleep. During this stage, the body hasn't fully relaxed, and the body and brain activities start to slow, with periods of brief movements. This stage usually lasts one to seven minutes, and people often believe they were awake during this stage.
Stage N2 is when the body starts to relax more deeply. The sleeper's heartbeat, breathing, muscle activity, and body temperature decrease, and eye movements stop. Dreaming is very rare during this stage, and the sleeper can be easily awakened. This stage is characterised by "sleep spindles", which are short bursts of high-frequency brain activity, and "K-complexes".
Stage N3, previously divided into stages 3 and 4, is deep sleep, slow-wave sleep (SWS). It is the most restorative stage of sleep, allowing the body to recover and grow. It is challenging to wake someone up during this stage, and if they do wake up, they may experience "sleep inertia," a state of confusion or "mental fog" that can last about 30 minutes.
Throughout the night, the duration of each NREM stage varies, and the time spent in each stage changes as individuals age. NREM sleep is vital for overall health, and sleep deprivation or fragmented sleep can reduce the time spent in NREM sleep, leading to health problems.
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NREM stage 1
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, with NREM stage 1 being the lightest stage of sleep. When a person first falls asleep, they enter NREM stage 1, which usually lasts for one to seven minutes, or five to ten minutes, making up about 2 to 5% of total sleep. During this stage, the body hasn't fully relaxed, and the brain and body activities start to slow down, with periods of brief movements. The brain activity associated with falling asleep in this stage is characterised by light changes.
During NREM stage 1, the body begins to prepare for deeper sleep. While the body temperature starts to drop, the heart rate and breathing remain relatively stable, with only minor reductions. The eyes are closed, but the individual is still easily aroused and can be awakened without much difficulty. This stage serves as a bridge between wakefulness and the deeper stages of sleep that follow.
As NREM stage 1 progresses, the brain waves slow down, marking the transition to NREM stage 2. This transition is characterised by noticeable pauses between short, powerful bursts of electrical activity in the brain. These bursts are believed to be the brain's way of organising memories and information from the time spent awake. NREM stage 2 is deeper than stage 1 but still considered light sleep, and it accounts for a significant portion of the total sleep time.
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NREM stages 2 and 3
Sleep is divided into four stages, including rapid-eye movement (REM) sleep and three stages of non-REM (NREM) sleep. These stages are determined based on an analysis of brain activity during sleep, which shows distinct patterns that characterise each stage. The breakdown of a person's sleep into various cycles and stages is commonly referred to as sleep architecture.
NREM stage 2 is a deeper level of sleep than stage 1. During this stage, individuals become less aware of their surroundings, their body temperature drops, and their heart and breathing rate slow down as their bodies prepare for deep sleep. This stage accounts for about 45% of total sleep time and usually lasts 10-25 minutes.
NREM stage 3 is the deepest and most restorative sleep stage, allowing the body to recover and grow. It is the most challenging stage to wake someone up from, and if they do wake up, they will likely experience sleep inertia, a state of confusion or mental fog that lasts about 30 minutes. This stage is crucial for feeling refreshed, and the body takes advantage of this very deep sleep stage to repair injuries and reinforce the immune system. During NREM stage 3, the body is in its most relaxed state, with the lowest heart rate and blood pressure. This stage makes up about 15-25% of sleep in adults, but the amount of time spent in this stage decreases as the night progresses.
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REM sleep
Sleep is broadly divided into two stages: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, while REM sleep is the fourth stage.
During REM sleep, the body experiences atonia, or temporary paralysis of the muscles, except for the eyes and the muscles that control breathing. This temporary paralysis is important to prevent people from acting out their dreams. The transition to REM sleep is marked by electrical bursts called "ponto-geniculo-occipital waves" (PGO waves) originating in the brain stem. These PGO waves are inferred to exist in humans based on similar effects observed during "phasic" events in REM sleep.
The duration of REM sleep varies with each sleep cycle, with the first cycle typically being the shortest, around 10 minutes, and subsequent cycles gradually lengthening, up to an hour. On average, REM sleep accounts for about 25% of total sleep time.
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Sleep cycles
Sleep is a vital process that allows the body to rest, repair, and restore itself. Sleep cycles are the different stages of sleep that the body transitions through during a night of sleep. These cycles are broadly divided into two types: rapid-eye movement (REM) sleep and non-rapid eye movement non-REM sleep, also known as non-REM sleep. Together, these two phases make up the four or five sleep cycles that a person typically completes in a full eight-hour sleep.
The first phase of sleep is non-REM sleep, which is composed of four stages. The first of these is N1, the lightest stage of sleep, which occurs when a person first falls asleep. During this stage, the body hasn't fully relaxed, and the brain and body activities start to slow down, with periods of brief movements. N1 usually lasts one to seven minutes.
The second stage is N2, where the body starts to relax more deeply. Body temperature drops, muscles relax, and the heart and breathing rate slow down.
The third stage is N3, which is deep sleep. This is the most restorative stage of sleep, allowing the body to recover and grow. During the early sleep cycles, N3 commonly lasts for 20 to 40 minutes. As the cycles continue, the duration of N3 gets shorter, and more time is spent in REM sleep.
The fourth stage is REM sleep, where most dreaming occurs, brain activity increases, and the body experiences atonia, or temporary paralysis of the muscles. The eyes, however, move rapidly behind closed eyelids, giving this stage its name. During REM sleep, the brain waves are similar to those during wakefulness.
After the first REM cycle, the sleep cycle repeats, starting with either stage N1 or N2. On a typical night, a person will cycle through four or five times, with each cycle lasting around 90 to 120 minutes.
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Frequently asked questions
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, from light sleep in stage one to deep sleep in stage three. During REM sleep, your eyes twitch, your brain is active, and your muscles become limp. Dreaming usually occurs during this stage.
The stages of sleep are determined by measurements of brain activity, eye movements, and heart rate. Sleep studies use sensors to record these measurements and classify sleep phases and stages.
On average, the body cycles through all stages of sleep about 4 to 6 times each night, with each cycle lasting around 80 to 100 minutes. During the early sleep cycles, the deep sleep stages can last for 20 to 40 minutes. As the night goes on, these stages get shorter, and more time is spent in REM sleep.
