The 4 Sleep Phases: what are they and what happens in each one?


Sleep is one of the most important things to maintain good physical and mental health.. While we sleep, our body performs many important tasks for our body.

Each phase of sleep has a function and a role in maintaining brain and cognitive functions. In some stages, cell and tissue repairs are also carried out, in addition to other tasks that keep the body healthy and prepare it for the next day.

Sleep can be broadly segmented into rapid eye movement (REM) sleep and non-rapid eye movement (non-REM or NREM) sleep. During sleep, the body experiences multiple sleep cycles. Each cycle includes four stages: three non-REM stages and one REM stage.

In a night of sleep, a person usually goes through 4 to 5 cycles of between 90 and 120 minutes in length, of non-REM and REM sleep. Sleep stages change throughout the night, with more non-REM sleep in the first half and more REM sleep in the second half.

In this article, we will discuss in detail the different phases of non-REM and REM sleep, including the physiological changes that are experienced in the body during each one.

  • We recommend you read: “The 10 consequences of lack of sleep on our health (physical and emotional)”

What is a sleep cycle?

Sleep is not a uniform process. Sleep cycles are made up of four stages, which are repeated throughout the night while the person sleeps. Each sleep cycle has a different duration, but the average is about 90 minutes. During a typical night, a person goes through between 4 and 6 sleep cycles.

You can see how the brain behaves when a person falls asleep using an electroencephalogram. This is a noninvasive test that records brain activity. During the initial stages of sleep, a person is still quite conscious, and alert. The brain produces beta waves: small, fast waves that show the brain is active and working.

As the brain relaxes and slows down, alpha brain waves begin to be recorded. During the transition to deep sleep, strange and unusual sensations, called hypnagogic hallucinations, are sometimes experienced.

When someone feels like they are falling, or hears their name being called, they are experiencing hypnagogic hallucinations. Myoclonic jerk is also a common example of this phenomenon, this refers to the sudden jump -for no apparent reason- that can occur while a person sleeps. Alcohol consumption, age, and sleep patterns affect the number of sleep cycles a person has per night.


What are the stages of sleep?

Sleep is a physiological process of our body that takes place while we sleep. Sleep, through its cycles, allows the brain and the rest of our body’s systems to carry out tasks necessary for the proper functioning of our body. These tasks include the repair of cells and tissues, the elimination of toxins and the processing of memories, among others.

While we sleep, the brain goes through different patterns of brain activity, which show the different stages of sleep a person is in and can be detected thanks to an electroencephalogram.

The amount of time spent in each stage of sleep and the number of sleep cycles change as the night progresses. The first sleep cycle is the shortestlasts between 70 and 100 minutes, while subsequent cycles are longer, lasting between 90 and 120 minutes.

In addition, the composition of each cycle, the time spent in each stage of sleep (REM or non-REM) also changes as the hours go by. There are three rapid eye movement (REM) stages and three non-rapid eye movement (NREM) stages. Here are the four stages of the sleep cycle:

1. Non-REM sleep phases

Non-REM sleep is divided into three different stages. The higher the non-REM sleep stage, the more difficult it is to wake a person.

1.1. NREM sleep stage 1

Wakefulness and sleep coexist for a short time at the beginning of the sleep cycle. The first phase of the sleep cycle is a transition between these two states. Stage 1 NREM is the shortest stage of sleep, lasting only 5-10 minutes.. During this phase of sleep, a person may think that he was not actually sleeping, if he wakes up.

During the N1 phase of sleep, the body does not completely relax, but the activities of the brain and body begin to slow down. There are small changes in brain activity during this stage of sleep: the brain produces more delta waves and some physiological changes occur:

  • The brain slows down, it can no longer process information as well.
  • Heartbeat, breathing, and eye movements slow down.
  • When the body is relaxed, the muscles can contract.
  • The brain produces slow waves in the front part of the brain

Approximately 50% of the total time spent sleeping is in stage 2 non-rapid eye movement sleep. This phase usually lasts about 20 minutes per cycle.

1.2. NREM sleep phase 2

When we go to sleep for the first time, our body slows down and prepares to enter phases 3 of sleep and REM, which are the phases of deep sleep in which the body and brain repair and adjust for the day Next.

sleep phase 2 it is also a stage where our body slows down, and occurs when the brain produces more Delta brain waves. Phase 2 of the sleep cycle brings the body into a more relaxed state, as for the other phases a series of physiological changes take place: the muscles relax, breathing and heart rate are slower and the temperature drops.

At the same time, the eyes stop moving and the brain produces new wave patterns. The level of brain activity decreases, but there are times of increased activity that help the person to combat arousal from external influences.

The N2 phase generally occupies about half of a person’s total sleep time. During the first sleep cycle, phase 2 can last between 10 and 25 minutes. Each phase can get longer as the night progresses.

1.3. NREM sleep stage 3

Stage 3 sleep is also known as deep sleep. In this phase of sleep, the body relaxes even more, the respiratory rate decreases, as does the heart rate. The body also releases more melatonin. All this makes it difficult for the person to wake up.

Stage 3 sleep is also called slow wave sleep, because the brain produces delta waves during this stage. Stage 3 sleep is important for restful sleep, allows the body and its tissues to grow and heal, supports the immune system and performs other key functions for the body. This stage of sleep also contributes to the maintenance of cognitive functions and memory.

During the first half of the night, more of the sleep time is spent in deep sleep phases. The N3 stages last between 20 and 40 minutes in the first sleep cycles. As you fall asleep, the N3 stages shorten and more time is spent in REM sleep.


2. REM sleep phase

During REM sleep, the body is paralyzed, except for the muscles responsible for breathing and the eyes. This state is known as atony. At the same time, brain activity increases, approaching the levels seen when a person is awake.

Although the body does not move, you can see that the eyes make movements, which is why this stage is called REM sleep. REM sleep is crucial for many cognitive functions, including memory, creativity, and learning. Increased brain activity is seen in REM sleep, which is the sleep phase with the most vivid dreams. Dreams can occur at any time in the sleep cycle, but are more intense and frequent in REM periods.

The amount of time needed to enter REM sleep stages changes throughout the night, depending on the amount of time asleep. Generally, 90 minutes of sleep is the earliest a person can enter REM sleep. As the night progresses, the time spent in REM sleep increases, especially in the second half of the night. In total, adults spend about a quarter of their sleep in REM sleep.

Sequence of sleep stages

Although we can assume that yes, sleep does not progress through the four phases described in a perfect order, there are some alterations that are important to describe. In a full night of uninterrupted sleep, the sleep phases progress as follows: the first three sleep phases if they occur in a sequential order, but at the end of NREM phase 3 it returns to NREM phase 2 sleep (before entering the REM phase).

After REM sleep is over, the body normally goes through NREM 2 before repeating the cycle again. The cycle repeats itself 4-5 times throughout sleep, and the time spent in each stage changes throughout the night.

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