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Sleep cycles with different sleep phases

Research into the phases of sleep began with something of a bang. In 1862, Ernst Kohlschütter investigated human sleep behaviour and rudely roused his test subjects from sleep with a sonic pendulum. Sometimes the sleepers would startle almost instantly, sometimes they would find it difficult to wake up despite the loud bang. In the course of the night, sleep was apparently of varying depths, with Kohlschütter believing he could recognise a typical pattern of different sleep phases. He was to be proved right.

Healthy, restful sleep has a certain architecture: Phases of varying depth of sleep follow each other and form a typical sleep cycle that repeats several times per night. Four to seven sleep cycles are completed in one night, each lasting about 90 to 110 minutes.1,2 Depending on the depth of sleep, the wake threshold is lower or higher.

In the meantime, the individual sleep phases have been researched in more detail using modern research methods. In the sleep laboratory, polysomnography can be used to measure and record a variety of body functions during sleep.

Encephalography – the recording of brain waves – in particular has provided important insights into the structure of healthy sleep. The electroencephalogram (EEG) maps the nerve activity in the cerebral cortex.3 This examination can be carried out while the patient is awake, but also during sleep in the sleep laboratory.

In an encephalography, electrodes are attached to the scalp with a contact gel. A head shave is not necessary, but the hair should be freshly washed. Cables lead from the electrodes to the EEG device, which records the registered brain waves.

The EEG of a healthy person has a typical profile. The brain waves run in waves of different frequency (frequency of the spikes) and amplitude (height of the spikes). Short, high-frequency waves characterise mental activity. Elongated waves are characteristic of deep sleep. The waking state and the different sleep phases have different brain wave patterns.

From light to deep sleep to dream sleep

After falling asleep, we slowly slide into a deeper and deeper sleep. 1.4 The phase of falling asleep (N1) is first followed by superficial sleep (N2), which turns into deep sleep (N3). These phases are grouped together as non-REM sleep to distinguish them from dream sleep – REM sleep – which ends the sleep cycle. Adults spend three quarters to three fifths of the night in light sleep, one quarter to one fifth in deep sleep and another fifth in REM sleep. In children, the proportion of REM sleep is significantly longer. 5

During the night, cycles of non-REM sleep and REM sleep repeat four to seven times. The deep sleep phases become shorter and shorter towards the end of the night. The REM phases, on the other hand, increase in length in the course of the night. Both – the complete relaxation in deep sleep, but also the processing of experiences in REM sleep – are crucial for the quality of sleep.

At a glance: The different sleep phases

Non-REM sleep (no rapid eye movements)

  • Stage N1: Falling asleep phase, half asleep
  • Stage N2 Light sleep
  • Stage N3: Deep sleep

REM sleep (with rapid eye movements, REM)

  • Dream Sleep

Falling asleep phase (N1): between waking and sleeping

Some people fall asleep after just a few minutes, for others it takes much longer. As a rule, the falling asleep phase lasts up to 30 minutes. Body and mind come to rest. Breathing and pulse become even, muscles begin to relax. The brain waves slow down. Increasingly, we become less sensitive to external stimuli.

During the phase of falling asleep, it often happens that the muscles of the legs twitch uncontrollably. This is because the brain shuts down faster than the muscles. It is also common to have the feeling of falling during this phase, which is due to adaptation difficulties of the organ of equilibrium. The slow rolling eye movements that may occur during falling asleep are not to be confused with the rapid eye movements of dream sleep.

Light sleep (N2): The musculature slackens


Immediately after falling asleep, sleep is initially still superficial. Sleep physicians refer to this phase of light sleep, which lasts around 30 minutes, as stage N2. Even now we are still relatively easy to wake up. Breathing and heartbeat are lowered even more in the light sleep phase and muscle tension continues to decrease. It also affects the muscles in the throat, causing some people to start snoring. In many cases, this is a nuisance, but not a health hazard. Obstructive sleep apnoea (OSA) is different, in which breathing stops repeatedly during sleep. If you snore loudly and feel tired during the day, you should suspect obstructive sleep apnoea, because there are effective treatment options.

Deep sleep (N3): crucial for regeneration

When we are in deep sleep (stage N3), nothing wakes us up easily. Deep sleep is used for recovery and regeneration. The brain, as the overriding control centre for all bodily functions, has switched to rest mode. Blood pressure drops and the heart beats more slowly. The tension state of the muscles is lowered even further and movements are stopped. This is how the body recovers from the efforts of the day.

REM sleep: Experience processing in dreams


In REM sleep, which follows deep sleep, muscle tone is reduced even further. Now the muscles are almost completely flaccid. At the same time, the brain leaves rest mode and shows signs of increased activity.6 The rapid eye movements behind closed eyelids, which give REM sleep its name, also testify to the fact that our brain is very active during this sleep phase.

During REM sleep we dream. If people are woken up during an REM phase, they are often torn out of wild, fantastic-looking dreams in glorious colours. The next morning, they usually can't remember these dreams.

Dreams are important to process the experiences of the day. Emotional processing is in the foreground, which is reflected in the very pictorial character of the dreams, which apparently emerge from deep, unconscious layers of the personality.

Especially in this day and age, a multitude of experiences – both positive and negative – assail us during the day, most of which remain unfiltered and unprocessed for the time being. It's only during REM sleep that we find the opportunity to deal with this. REM sleep is therefore of vital importance for our psychological well-being.

Learning while you sleep – there’s something to it

In addition, REM sleep is associated with learning and remembering. So putting the maths book under your pillow at night actually does something? Of course, it’s not quite that simple. Nevertheless, there is something to learning while you sleep.

This is shown by a simple experiment: First, you have test subjects learn new word pairs. One group is allowed to sleep afterwards, while the other has to stay awake for the night. The next day, the newly learned word pairs are then tested and the same thing always emerges: Sleepers are significantly better at remembering what they have learned than non-sleepers.7

During sleep, memory traces formed the day before are apparently trained and thus consolidated.8 It is possible to visualise patterns of activity in the brain and show that new memory traces ‘light up’ repeatedly during sleep.9 Both deep sleep and REM sleep seem to be involved in the consolidation of learning content.10,11 The fact that sleep has an impact on memory performance is also indicated by a study showing that too little REM sleep increases the risk of dementia.12

Too little REM sleep, higher mortality

More seems to happen during REM sleep. There is no other explanation for the fact that too little REM sleep is associated with increased mortality. This is the result of two long-term studies, the results of which were published in 2020. Thousands of Americans were observed over up to twenty years, with regular examinations in the sleep laboratory. In people with shortened REM sleep, mortality due to cardiovascular complications was particularly increased.

Obstructive sleep apnoea (OSA) – disturbed sleep architecture with consequences 

An intact sleep architecture is crucial for restful sleep, with deep sleep and REM sleep being of particular importance. If the sleep architecture is permanently disturbed, sleep loses its recovery function. As a result, those affected – even if they sleep long enough – do not feel rested in the morning and are constantly tired during the day.

Chronic daytime sleepiness is typical of obstructive sleep apnoea (OSA), in which breathing stops repeatedly during sleep. If left untreated, OSA is associated with health risks such as high blood pressure and heart disease. It seems to indicate that the breathing interruptions repeatedly lead to interruptions of the important deep sleep and REM sleep phases. However, the connections have not yet been clarified down to the last detail. Research into sleep therefore remains highly exciting and further findings can also be expected with regard to obstructive sleep apnoea.

What could also be of interest to you:

Tips for falling asleep

Consequences of sleep disorders

Information material


1 University Hospital Regensburg. Available online at www.ukr.de/kliniken-institute/innere-medizin-2/Medizinische_Leistungen/Schlaflabor/­Gesunder_Schlaf/; Last accessed: November 2020.
2 Zulley J. The physiology of sleep. Available online at epub.uni-regensburg.de/19970/1/­ubr07567_ocr.pdf; Last accessed: November 2020.
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11 Diekelmann, S., Steiger, A. Memory and sleep. Somnology 2016 20, 6-7. Available online at link.springer.com/article/10.1007/s11818-016-0040-1; Last accessed: November 2020.
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