THE PHASES OF SLEEP AND THEIR CHARACTERISTICS - PSYCHOLOGY - 2025

The phases of sleep are one of the components of our biological clocks, a series of cycles that occur repeatedly during our lives. Specifically, these stages are different types of brain patterns that occur while we are sleeping. Each of them lasts between 90 minutes and two hours, and they all fulfill fundamental functions for our rest and health.

There are five main phases of sleep: wakefulness, relaxed wakefulness, light sleep, deep sleep, and REM sleep. Due to the duration of each of them, during a full night of sleep we usually go through between four and six of them.

Understanding the phases of sleep, their operation and their duration is essential, because waking up in the middle of a cycle can cause totally different effects than what happens if we wake up during another. The end of REM is generally considered to be the best time to give up sleep.

In this article we will study the five main sleep phases, their characteristics and the importance of each of them.

The phases of sleep

Sleep hypnogram between midnight and 6.30am Source: I, RazerM

Normally, when we think about the sleep cycles we go through each day, we get the impression that there are only two different states: wakefulness and sleep. However, the reality is a bit more complex. Studies carried out with modern neuroimaging techniques have shown that sleep is in turn divided into two very different types of phases.

Thus, while we are sleeping, the phases we go through can be found within two different categories: several stages in which dreams do not occur, and one of them in which they do. On a technical level, therefore, the different parts of the diurnal cycle are the waking phases, non-REM sleep phases, and REM sleep phases.

REM phases vs non-REM phases

Electroencephalography of a mouse. REM sleep is characterized by a prominent theta rhythm. Source: Andrii Cherninskyi

Despite the time that we have been researching sleep, the truth is that we know quite little about it. However, this activity is essential for our survival, and we know that all mammals and birds also sleep. In addition to this, the rest of the species in these categories also share the division between REM and non-REM phases.

But how are the two categories different? On the one hand, non-REM sleep is characterized by a relative lack of activity in the brain, and the possibility of moving our body while we are in it. Sleep of this type is divided into several phases depending on how deep the person's relaxation is, and how difficult it is to wake them up at each moment. Furthermore, dreams do not occur in this category.

On the other hand, we also find the REM phase. The most striking feature of it is the presence of dreams, a series of images that are very vivid and apparently related to the circumstances of our life, although populated with unreal elements.

This phase receives its name from the acronym in English for rapid eye movements, or rapid eye movements. This is because when we are in REM, our eyeballs are racing underneath our closed eyelids. It is believed that this movement has to do precisely with the formation of dreams.

In addition to this, during the REM phase our body cannot move by itself, but is paralyzed. Researchers think that this occurs so that we cannot put ourselves in danger while we are dreaming.

Phases of the sleep-wake cycle

Phase 1: Light sleep

The first phase of sleep is also known as the light sleep stage. It is one of the shortest duration, since it usually only lasts between five and ten minutes. It is the phase that serves as an intermediary between the state of being awake, and the deeper sleep that occurs in the following phases.

In phase 1, both the mind and the body begin to slow down their functions, so during it we feel relaxed and as if we were dizzy. During this stage we are not really asleep, so it is very easy to return to a waking state if someone tries to wake us up.

In fact, it is this ease of waking up during the light sleep phase that means that naps should not last longer than twenty minutes. After this time, the most common is to have passed to the stage of deep sleep, which causes that when we wake up we find ourselves disoriented and wanting to continue sleeping.

Phase 2: medium sleep

During the second phase of sleep, we are still considered to be in a relatively light state of sleep. However, both our brain waves and body activity and eye movement begin to slow down and prepare to enter a much deeper state of rest.

In this second phase of sleep, the brain produces sudden increases in its activity, which are seen in the shape of spikes on an EEG. Although it is not very well known what function they perform, it is believed that they are related to the creation of new memories and the processing of sensory information.

This stage is one of the most important in the entire sleep-wake cycle, since experts believe that it is there that long-term memory is consolidated.

Phases 3 and 4: deep sleep

These two phases of sleep are usually studied together, to the point that they are sometimes grouped into a single stage known as the deep sleep stage. However, there are some differences between them that cause most experts to decide to treat them as two separate phenomena.

One of the most important characteristics of the phases of deep sleep is that when we enter one of them, it is much more difficult for us to wake up. In fact, if someone takes us out of our rest at this moment, it produces what is known as "sleep inertia": the feeling that we are not fully awake, with effects such as dizziness, difficulty thinking and feeling tired.

During the phases of deep sleep, our muscles are completely relaxed. In addition, some functions such as breathing, temperature regulation or the speed of the heartbeat slow down to a great extent when we enter them.

Likewise, in the stages of deep sleep the body begins to produce a greater amount of growth hormones, and is responsible for several functions related to the maintenance of body well-being, such as the creation of new muscle tissues or the regulation of the immune system.. Because of this, these phases are especially important for our overall health.

Phase 5: REM sleep

Polysomnographic record of REM sleep. Lines in red represent rapid eye movements. Source: MrSandman at English Wikipedia

After the phases of deep sleep, if we continue to sleep, our body and brain enter what is possibly the best-known stage of the entire cycle: the REM (rapid eye movement) phase. When we are in it, many of the characteristics of the other stages are completely reversed, passing our state to a much more similar to that of waking.

For example, during the REM phase both the speed of our heartbeat and our blood pressure increase significantly, especially when we compare them with their state during the deep sleep phase. Furthermore, our breathing tends to become irregular, rapid and shallow; and our whole body regains the ability to move freely.

Dreams

At this stage is when we dream; in fact, externally the REM phase is distinguished by the rapid movement of the eyes. In it, the brain is activated in the same way that it does in reality. For example, if basketball is played in the dream, the activated brain areas would be the same as if it were played in reality. Below you can see how the eyes move in this phase:

Brain changes

On the other hand, the most important change that occurs during the REM phase occurs at the brain level. If we measure its activity with an encephalogram, this organ shows a very significant increase in its activity, which normally correlates with the appearance of dreams.

The functions of the REM phase are still largely a mystery to scientists, and therefore are still being studied. However, this stage is believed to play a very important role in the mind's ability to learn and memorize information, in a process that works in conjunction with that of the deep sleep phase.

It has been proven that the REM stage occupies approximately 30% of all our sleep time, and is one of those that has the greatest impact on our feeling of tiredness. Therefore, waking up in the middle of this phase tends to produce higher levels of physical and mental exhaustion.

What are dreams for?

In addition to fulfilling several very important functions for our rest and the functioning of our brain, the REM phase is also famous because dreams appear during it. However, research on this phenomenon has not yet clarified what its purpose is or how exactly they work.

Some psychological theories affirm that dreams help us discharge different tensions that we face in our day to day life, or that they prepare us to overcome the challenges that are presented to us. Other experts, however, claim that they are completely arbitrary.

In any case, it has been proven that if we do not spend enough time in REM sleep, our levels of fatigue increase, even if we are sleeping for a long time. This can happen, for example, if we consume substances that affect the natural sleep cycle, such as alcohol, coffee or tobacco.

References

1. "Understanding Sleep Cycles: What Happens While You Sleep" in: Sleep. Retrieved on: November 09, 2019 from Sleep: sleep.org.
2. "The stages of sleep" in: Sleep Cycle. Retrieved on: November 09, 2019 from Sleep Cycle: sleepcycle.com.
3. "Stages of Sleep and Sleep Cycles" in: Tuck. Retrieved on: November 09, 2019 from Tuck: tuck.com.
4. "Stages of Sleep: The Definitive Guide" in: Oura. Retrieved on: November 09, 2019 from Oura: ouraring.com.
5. "What Are REM and Non-REM Sleep?" in: WebMD. Retrieved on: November 09, 2019 from WebMD: webmd.com.