Sleep directly impacts our bodily functions and how our brain performance. Each sleep stage plays a critical role in our body’s overall maintenance and performance. The sleep cycle also ensures the repair and maintenance of various body parts while the body is at rest. The sleep cycle can be classified into 4 stages. Our brain cycles through these stages of sleep as we are sleeping. 

• Stages from 1 to 3 are considered non-rapid eye movement(NREM) sleep or quiet sleep.
• Stage 4 is considered a rapid eye movement(REM) stage, which is also known as paradoxical sleep

How does the sleep cycle work?

This whole cycle of sleep repeats itself after each successful REM stage. The sleep cycle repeats itself by increasing the sleep duration and intensity of the sleep.

In the earliest stages of sleep, our body is relatively awake and alert. During this time, our brain produces beta waves. These signals are small and fast brainwaves that are associated with mental and intellectual activeness. 

Gradually, as we move to deep stages of sleep, our brains start relaxing and downturns activities. At this stage, our brain produces alpha brainwaves, that promote emotions of calmness, boost creativity, and improve your capacity to learn new things. With this, we can also experience unusual and lucid hallucinations called hypnagogic hallucinations.

A sudden sensation of falling and wakefulness are usually a part of this phenomenon while falling asleep. 

NON REM Sleep Stage 1

The first stage of sleep represents the transition period – between which the body is transitioning from wakefulness to sleep.

During this stage, our body experiences the following sensations:

• The brain slows down its activities 
• The heartbeat, eye movement, and breathing slow down as well
• The body goes into a relaxation phase and our muscles may twitch

This period lasts for about 10-15 minutes. However, the brain is still active and produces theta waves. 

NON-REM Sleep Stage 2

This stage of the sleep cycle lasts longer than any other stage. NREM stage 2 takes a time duration of 20 minutes of our sleep cycle. Throughout the sleep duration, about 50% of the time is spent in this stage.

You can experience the following at this stage of your sleep cycle:

• You feel drowsy and are less aware of your surroundings
• The body temperature lowers
• Your eye movement hauls 
• Your heartbeat and breathing become more stable

During this time, our brain produces rapid and rhythmic waves, also known as sleep spindles. The working of sleep spindles are complex brain functions that indicate the body is preparing for the next stage of sleep.

NON-REM Sleep Stage 3

NREM Stage 3 is denoted by delta waves which are slow and deep brainwaves. This is the stage of sleep where external disturbances or noise may fail to bring a sleeping person back to wakefulness. 

Getting enough sleep during this stage ensures that we wake up refreshed and recharged in the morning. 

During NREM stage 3: 

• Your muscles are fully relaxed
• The blood pressure level lowers 
• Your breathing is slower 
• You start progressing towards deep sleep

The body starts maintaining and repairing itself while our brain is busy consolidating memories like – metrics or facts, what you learned throughout the day, general knowledge, and memories related to these. 

REM Sleep Stage 4 

While our brain is going through a rapid eye movement stage, our brain’s activity closely resembles the activities in our waking hours. At this stage, we experience vivid dreams. During this time, however, our body is immobilized which prevents us from acting out our dreams.

During REM Stage:

• You tend to dream
• Your brain initiates its activities 
• Your body is paralyzed and relaxed
• Your eye moves rapidly

In this stage, memory consolidation takes place – most of the memories associated with emotions are processed and stored. These memories are then cemented into our subconscious memory as an inevitable part of our brain’s database. 

However, we have to take into account that all 4 stages of the sleep cycle maybe not follow the same sequence. Interruption during sleep can affect the sequence and will reinitiate these stages. Moreover, our sleep cycle gets repeated at least 4 to 5 times during sleep, and the time spent in each stage can also vary. 

Source: 

The 4 Stages of Sleep, July 15, 2022, Verywellhealth

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