Our Sleep Cycle
Our sleep consists of various phases and stages, which repeat in the same order several times per night. The progression of the individual sleep phases is not random, but follows a recurring pattern known as the sleep cycle. A sleep cycle usually lasts about 90 – 110 minutes and repeats 4 – 7 times per night. If there is a balanced ratio of all sleep stages, we perceive the sleep as restful.
The American Academy of Sleep Medicine divides the sleep cycle into 2 phases: Non-REM sleep and REM sleep.*A 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.
Deep sleep (Stage N3)
Now follows the deep sleep phase, in which we sleep deeply and soundly and can only be awakened with difficulty. This phase lasts about 20 – 30 minutes. Breathing is now very regular, muscles relax and brain activity drops to a minimum. Deep sleep is particularly important for our physical and mental recovery and regeneration.
Our tip
If you are awakened during the deep sleep phase, there is the greatest risk of feeling tired and groggy. An alarm clock with sleep phase alarm function recognises a "nearly awake moment" and orientates itself accordingly. This makes getting up easier.
REM Phase
Every 60 – 90 minutes, REM sleep occurs: the eyes move rapidly, the brain is very active and breathing becomes irregular. Characteristic of this phase are the rapid eye movements under the closed lids.
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.
In which sleep phase do we dream?
In principle, dreams can occur in any phase of the sleep cycle. However, we dream very regularly and intensely in the REM phase, which is why it is also referred to as the dream phase. If we are awakened during REM sleep, we often remember the dreams very clearly.
Did you know?
Based on an average lifespan, we spend about six years dreaming. Just five minutes after getting up, we have already forgotten 50% of our dreams, after ten minutes it's already 90%.
*A https://aasm.org/clinical-resources/scoring-manual
*B https://www.cardiopraxis.de/koennen-sie-einen-schlafmangel-aufholen/
*C Dr. Michael Feld, Schlafen für Aufgeweckte, S. 25