What is the Physiology of Sleep

What is sleep

A good night's sleep is the basic requirement for mental and physical well-being. Humans spend about a third of their lives sleeping.

For a long time, sleep was viewed as an inactive human condition, as a special position between life and death. Aristotle, for example, defined sleep as the opposite of being awake. While the god Hypnos was responsible for sleep among the Greeks, the Roman Galen (129-199 AD) recognized that this is controlled by the brain. In this way the opinion was established that sleep serves to regenerate the brain.

It was only with the invention of electroencephalography (EEG) and the associated possibility of measuring brain activities - waves with different frequencies and amplitudes - that science recognized that sleep is a dynamic process of its own, with special brain activities triggered by different mechanisms regulated and controlled.

Why do we have to sleep?

Even today, however, science still leaves an open answer to the question of the function of sleep. What we now know - unfortunately many from our own experience - is that sleep deprivation noticeably reduces the quality of life and can subsequently lead to illness and, in extreme cases, death. Sleep is usually seen as a natural process - how important it is for well-being is only realized as soon as sleep disorders occur.

++ More on the topic: Sleep disorders - what to do? ++

Sleep as relaxation for organs

Thus theories emerged that ascribe an energy-conserving effect to sleep - energy consumption drops by around 10% during sleep - or see it as an important recreational factor for various organs.

Sleep for brain development

Other theories view sleep as a necessary part of brain development or as a way to give the brain time to separate important information from unimportant information - to prevent "overloading".

Sleep therefore represents a dynamic process in the brain that is guided by special nerve cells - neural circuits at the level of the brain stem, the center for non-arbitrarily controlled processes in the body.

Here, on the one hand, there is control over the beginning and end of sleep in harmony with day and night, but also regulation of the sleep cycle, which is characterized by a sequence of regular changes in the activity of the brain. Sleep is therefore not a uniform state, but is made up of individual, cyclically recurring phases.

Sleep phases

This is made clear by the observation that at certain intervals there is eye movement during sleep, which is clearly visible through the eyelids. If you wake the sleeper in this phase, he usually reports of dreams he has just experienced. This phase is called REM phase (Rapid Eye Movements).

The rest of the time is called NonREM sleep (Non Rapid Eye Movements). With the help of the EEG, this is divided into four phases:

Stage 1

In this section, also known as the sleep phase, the beta waves that are present in the waking and alert state are first replaced by alpha waves. The person is relaxed but not yet asleep. Only the appearance of theta waves heralds the transition to sleep. The beginning muscle relaxation ("nodding") is often initiated by muscle twitching (myoclonus), which startle the person asleep for a short time.

Stage 2

If sleep spindle and K-complexes are now found in the EEG, one speaks of the second stage. At this stage, as in the previous stage, the muscle tone (tension) is not yet completely reduced. The person can still be woken up easily.

Stage 3 and 4

Now the deep sleep begins, from which it is difficult to wake the sleeper. Slow delta waves now predominate in the EEG, the mucosal tone is significantly reduced.

After a brief re-entry into stage 2, the eye movement typical of the REM phase occurs. This ends the first sleep cycle, which usually lasts 90 minutes. This cycle is repeated about 4 to 5 times per night, with the deep sleep stages predominating in the first half of sleep and the REM phases in the second half.

Physical changes during sleep

Sleep is a pronounced rest phase with hardly any motor activity and very little willingness to react to external stimuli. Nevertheless, it is not a state of complete passivity, because highly complex processes take place in the brain even during sleep.

The physiological changes during sleep affect almost all organ systems and depend on the various phases of sleep:

  • For example, the heart rate and blood pressure decrease in non-REM sleep and increase again in the REM phase.
  • The body temperature drops during the night and many hormone systems are also influenced by the sleep-wake cycle.
  • Some hormones are released during sleep (e.g. growth hormone), while others are suppressed during phases of deep sleep (e.g. cortisol).
  • As soon as sleep is interrupted or ended, consciousness returns immediately and all body functions adapt to the waking state again.

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Dr. Erich Mayer-Fally (2002)
Editorial editing:
Dr. med. Kerstin Lehermayr

Updated on:

Stuck B. A. et al: Practice of Sleep Medicine; Springer Medizin Verlag Berlin Heidelberg; 2nd edition 2013

Lautenbacher S., Gauggel S .: Neurophysiology of mental disorders; Springer Medizin Verlag Berlin Heidelberg; 2nd edition 2010

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