The Sleep Phases – How important they are for our recovery and regeneration

Table of Contents

1. Our Sleep Cycle
   
   
2. How can we determine the individual sleep stages?
   
   
3. Non-REM Stage
   
   
4. REM Stage
   
   
5. In which sleep stage do we recover particularly well?
   
   
6. In which sleep stage do we dream?

The American Academy of Sleep Medicine divides the sleep cycle into 2 phases: Non-REM sleep and REM sleep.^*1 The Non-REM phase can be further divided into 3 stages: Sleep onset stage (N1), light sleep (N2) and deep sleep (N3). The term "REM" stands for "Rapid Eye Movement" and describes the rapid eye movements with closed eyelids that are characteristic for the REM phase.

In the first half of the night, Non-REM sleep predominates, in the second half of the night REM sleep prevails.

How can the individual sleep phases be determined?

In a sleep laboratory, brain, eye and muscle activity during sleep can be measured using an electroencephalogram (EEG). Electrodes are placed on the scalp to record the brainwave patterns during sleep. Based on these wave-like patterns, sleep researchers can identify and analyse the different phases.

Non-REM Phase

Sleep onset stage (Stage N1)

The sleep onset phase is the transition phase from wakefulness to sleep and ideally lasts about 5 – 15 minutes. In this phase, your body prepares for sleep. Body temperature drops, breathing and heartbeat regulate themselves and brain activity slowly decreases. At the same time, our muscles start to relax, which sometimes manifests in muscle twitches. In this phase, you can often still perceive noises and disturbances from the outside.

The EEG records increased alpha waves during this phase. They are comparable to a state of relaxation or a light daydream.

Light sleep (Stage N2)

Once we have overcome the sleep onset phase, we enter the light sleep phase. With a duration of 40 – 50 minutes, it accounts for about 50% of the sleep cycle. Now the actual sleep begins. Body and mind continue to relax: muscle tension gradually decreases and brain activity decreases. Perception of external stimuli is also reduced. However, the wake threshold is still relatively low, meaning we can be easily awakened during this phase.

The EEG now shows theta waves, which have an even lower frequency than alpha waves. At the same time, sleep spindles and K-complexes occur more frequently. Sleep spindles are short bursts of fast, rhythmic brain waves, while K-complexes are large, wave-like deflections.

Increased growth hormones are released, which are of particular importance for physical regeneration. Tissue and cells are renewed, the immune system is strengthened and energy for the coming day is recharged.

In this phase, the very slow delta waves dominate the EEG. They have the lowest frequency of all brain waves and are characteristic of deep relaxation and recovery.

REM sleep is also known as the dream phase, as we dream particularly intensively here. In this way, our brain processes experiences and information from the day. At the same time, cognitive processes such as learning and memory formation are stimulated. You can imagine the whole thing as a kind of mental desk. During the day we experience many different things, which we file as notes on our mental desk. In REM sleep, these note papers are sorted, discarded and stored, so that we can access them again later.^*2

During the REM phase, muscle activity is greatly reduced. A protective mechanism of the body that protects us from executing real movements. Otherwise, we would run the risk of injuring ourselves or others during intense dreams.

The EEG measurement in the REM phase is similar to that of the waking state.

In which sleep phase do we recover particularly well?

The first part of our total sleep duration is of particular importance for our vitality. After the first sleep cycle (sleep onset phase – light sleep – deep sleep – REM sleep), one to two more sleep cycles follow, after a possible short awakening. These cycles together are referred to as "core sleep". It is particularly important for our mental and physical recovery and regeneration.

Following the core sleep are 2 – 4 cycles of "fill sleep" or "optional sleep". The deep sleep phases in these cycles are usually only of short duration or completely absent, which is why they are of less relevance for our health.

^{*1 https://aasm.org/clinical-resources/scoring-manual
*2 https://www.cardiopraxis.de/koennen-sie-einen-schlafmangel-aufholen/
*3 Dr. Michael Feld, Schlafen für Aufgeweckte, S. 25}