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

Sleep is the most crucial element in optimal physical and mental health. While we sleep, our body performs many important tasks for our body.
Each sleep phase has a function and a role in maintaining brain and cognitive functions. In some states, 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 one sleep, a person typically goes through 4 to 5 cycles of 90 to 120-minute durations 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 piece, we will discuss in detail the different phases of non-REM and REM sleep, including the physiological changes experienced in the body during each one.
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 typical 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 various 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 shortest, lasting between 70 and 100 minutes, while subsequent cycles are the most extended, lasting between 90 and 120 minutes.
In addition, the composition of each cycle and the time spent in each stage of sleep (REM or non-REM) also change as the hours go by. There are three stages of rapid eye movement (REM) and three non-rapid eye movements (NREM or non-REM).In bellow, four phases of the sleep cycle are listed:
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 briefly at the beginning of the sleep cycle. The first sleep cycle phase is a transition between these two states. Stage 1 NREM is the shortest stage of sleep, lasting only 5-10 minutes. During this sleep phase, a person may think they were not sleeping if they wake up.
During the N1 phase of sleep, the body does not completely relax, but the brain and body activities begin to slow down. There are minor 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.
Muscles can contract when the body is at ease.
The brain produces slow waves in the front part of the brain.
Approximately 50% of the 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.
Stage 2 sleep 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, physiological changes occur: 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 moments of increased activity help the person 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 step can get longer as the night progresses.
1.3. NREM sleep stage 3
Deep sleep is another name for stage 3 sleep. In this sleep phase, the body relaxes even more, and 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 called slow wave sleep because the brain produces delta waves during this phase. Stage 3 sleep is essential for restful sleep; it allows the body and its tissues to grow and heal, support the immune system, and perform other vital functions for the body. This stage of sleep also contributes to the maintenance of cognitive functions and memory.
During the night’s first half, more 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 steps shorten, and more time is spent in REM sleep.


REM sleep phase
The only muscles active during REM sleep are those that control respiration and vision. 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, 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 in the REM phase increases, especially in the second half of the night. Adults experience REM sleep for around a quarter of their sleep time.

The sequence of sleep stages
Although we can assume that sleep does not progress through the four phases described in perfect order, some alterations are essential to describe. In a whole night of uninterrupted sleep, the phases go as follows: the first three sleep phases if they occur in 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, the body typically goes through NREM 2 before repeating the cycle. The cycle repeats itself 4-5 times throughout rest, and the time spent in each stage changes throughout the night.