Stages Of Sleep: Understanding The Sleep Cycle

The sleep cycle is made up of successive stages of changes in the brain's electrical activity.

Generally speaking, we can identify five stages of sleep.
Generally speaking, we can identify five stages of sleep.

We usually believe that the stages of sleep are marked by episodes of cerebral relaxation. The reality, however, is that instead of plunging into a state of relaxation and temporary inactivity, during sleep the human brain goes through a series of constant stages that are common in most mammals.  

This article aims to discover what the stages of sleep are and understanding the cycle of sleep. 

What is dreaming?

Before diving into the stages of sleep, we would like to look at the definition of dreaming. According to the American Heritage Dictionary of the English Language, psychologically speaking, dreaming represents a series of images, ideas, emotions, and sensations occurring involuntarily in the mind during certain stages of sleep.

Therefore,  dreaming is marked by events or images that manifest themselves in the fantasy of someone who is asleep. This characterizes sleep via a mixture of sensory experiences that take place while we sleep and that is related to our fantasies, desires, and fears. 

Stages of sleep: a short definition 

The stages of sleep are successive stages or steps with distinctive electrical and sensorial traits that humans go through while sleeping. 

These steps can be studied and measured precisely because they have specific traits regarding their biological characteristics and overall duration. 

Said traits can be studied and they help us obtain valuable information about the brain's mechanism. Sleep stages analysis can be done using specific medical tools such as  the electroencephalography (EEG)  - measures brain wave activity, and the electromyography (EMG) - measures muscle movement,  the electrooculogram (EOG)  - measures eye movement  and heart rate monitors. 

Up to a certain extent, these tools allow us to tap into the brain's activity and determine which stage of sleep it's in. The ability to evaluate a sleep cycle can also be useful in the case of treating related sleep pathologies.

An electroencephalogram (EEG) is used to measure the brain's electrical activity during sleep.
Electroencephalography (EEG) is used to measure the brain's electrical activity during sleep.

General classification: REM and non-REM sleep 

Sleep is one of the vital activities humans need to perform to stay healthy. 

A natural sleep cycle is based on a series of transitions from non-REM sleep to REM sleep, which represent the two main subdivisions of the sleep cycle. Each of them has specific characteristics and differences which we will explore in the following. 

1. Non-REM sleep 

Non-REM sleep, also known as  slow wave sleep, can be found in the first four stages of sleep and it represents the majority of our sleep cycle which makes up 80% of the time we spend asleep. 

Moreover, this stage of our sleep is characterized by slow brain waves, especially alpha, delta, and theta waves, as well as an increase in blood pressure. On the other hand, during non-REM sleep, we switch between the different phases of the cycle until we reach the most profound state of relaxation; which is where we truly get a good night's rest. 

2. REM sleep 

The REM sleep stage (Rapid Eye Movement) is the final one of the cycle and has the following attributes: 

  • Lighter sleep compared to the first four stages
  • Rapid and random eye movement
  • Low muscular activity 
  • Intense dreaming
  • Cardiac frequency
  • Irregular breathing
  • Irregular body temperature 
  • Nocturnal penile tumescence (a spontaneous erection of the penis during REM sleep) / Nocturnal clitoral tumescence (engorgement of the vagina during REM sleep)

REM sleep is also known as a paradoxical sleep phase or and sometimes desynchronized sleep because of physiological similarities to waking states, including rapid, desynchronized brain waves, without, however, the ability to move. 

The REM sleep stage is one of the most important for our body's optimal functioning and seeing how it is the stage where we dream; it's also the stage that consolidates any learning made during the day. From this perspective, REM sleep and dreaming seem to have an adaptive function that helps consolidate our memory. 

How many stages of sleep are there?

After identifying the two main categories of sleep, we need to have a look at the nature of the different sleep stages and their characteristics. 

1. The process of falling asleep 

This is the initial sleep stage and lasts a relatively short period, around 5% of the total amount spent sleeping. This stage is also a transition phase between wake and sleep. This is where we come across light theta waves, although if we were to perform an EEG, we would also find incipient alpha waves. 

In this first stage we experience very light sleep and even the slightest disruption can wake us. In reality, this phase is not exclusive to sleep as we also go into this stage when we relax during the day and we close our eyes thus inducing a state of calm.  

2. Stage II: light sleep

This sleep stage, known as light sleep, is the one where the body slowly disconnects from the brain. Something we tend to experience before we fall asleep is the sensation of falling, which is the brain's way of making sure that the areas responsible with regulating autonomous movements and activating the consciousness are functioning properly.

This stage is another popular one in our sleep cycle as we spend in it almost 50% of the total sleeping time. The theta waves decrease, the muscles relax, the cardiac rhythm decreases, as well as breathing.

3. Stage III: preparation for deep sleep

Phase three of our sleep cycle is considered by many to be an extension of stage four. Here, delta waves take over our brain, and the body prepares for deep sleep.

This is a short period, and it is regarded as a transitory stage toward step four. 

4. Stage IV: delta waves 

The delta stage or the deep sleep stage, as it is also called, is the most essential sleep stage in our sleep cycle. The name of this sleep stage is given by the presence of slow, intense delta waves which are associated with deep sleeping. 

This is when the body truly gets its rest and regenerates its vital energy. 

5. REM sleep

Finally,  the REM sleeping stage is marked by intense cerebral activity present in the brain. As we detailed earlier, dreaming happens during the rapid eye movement stage as well as internalizing any learning we did during the day. 

This phase is characterized by the inability to move one's body and has been related to lucid dreaming or lucid nightmares. 


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