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Healthy sleep

Mystery of sleep

Why do we sleep? This is a question which has occupied mankind since time immemorial. Brain research has meanwhile developed some plausible theories – and in doing so found out when and why things that you learn by heart stick in your mind for ever.

When you sleep badly or don’t get any sleep at all, you often feel absolutely shattered next morning: You perform at a much lower level, can hardly concentrate, and are more easily irritated. A common assumption therefore used to be that sleep was necessary so that the whole organism could regenerate itself.


Dangerous phase without consciousness

Gefährliche Phase ohne Bewusstsein

But this is actually illogical as far as evolutionary theory is concerned. Organs and muscles can theoretically also regenerate themselves in an awake state of rest. And a living being which sleeps and withdraws from consciousness is easy prey for predators. Despite this, vertebrates have evolved such that they sleep at regular intervals.
Sleep must therefore involve a crucial benefit. If the processes which then occur in the brain are analyzed in more detail using an EEG, the issue becomes clearer: The brain is very active during sleep - in all phases of sleep: deep sleep, REM sleep, and the various stages of a light, shallow sleep.

Completely different brain activity

Völlig andere Hirnaktivität

Typically, a night’s sleep starts with light, shallow sleep, during which the brain waves become slower and slower. As soon as consciousness fades, clearly measurable deflections become visible on the EEG again and again, the so-called sleep spindles and K-complexes. And during the transition to deep sleep, the brain wave frequency decreases significantly yet again. The brain therefore follows a completely different rhythm than during the waking state – and it is possible to observe that the nerve cells or neurons synchronize. They “send” and “receive” to a common beat.

So-called REM sleep represents a special phase of sleep. REM stands for Rapid Eye Movement. Although this sleep is similar to the light sleep which occurs shortly after falling asleep as far as the brain waves are concerned, rapid , non-directional movements of the eyes below the eyelids occur. Blood pressure, breathing, and heart rate increase. But the muscles are at maximal relaxation, with the exception of those controlling the eye balls. The individual phases alternate several times during a night’s sleep, the duration of each of the phases becoming shorter and shorter towards the morning, until we finally wake up.

Memory is generated during sleep

Im Schlaf entsteht das Gedächtnis

The EEG clearly demonstrates: The brain is very active during sleep, albeit that the activity is completely different to that in the awake state. During sleep, specific electrical currents are generated in the brain. And proteins form so that the nerve cells can link up with the aid of the so-called synapses.

There are many indications that the memory is generated during sleep – in a very real way in the form of newly connected nerve cells.This can be imagined quite illustratively, because as in a computer, there are different areas of the brain which are responsible for different tasks. The hippocampus at the very center serves as working memory for everything we experience during the day. New nerve cells and nerve connections are being formed continuously there. This means it can take in information particularly easily. But the information stored in the hippocampus can quickly be overwritten again.

This is in contrast to the neocortex, i.e., the strongly structured surface of the cerebrum. This is where the information from the hippocampus is transferred to and temporarily stored, until it is finally moved to the prefrontal cortex, our long-term storage unit. This transfer of information from one area to another takes place during sleep – controlled by the rhythmic, long wavelength brain waves.

Clear-out in the brain

Entrümpelung des Gedächtnis

This also involves an evaluation of the information, because not everything we experience every day is relevant for long-term storage. As part of the data transfer, the fast internal memory of the hippocampus is thus emptied as well. The brain itself has a clear-out, so to speak, and becomes receptive again for the next day. This explains why psychological tests have shown that things learned by heart stick best when you lie down and go to sleep immediately afterwards. And everybody knows from their own experience that, conversely, you can take in more information when you are well rested than when you are overtired.

The fact that the transfer of information from one part of the brain into another one plays a crucial role here, is a quite new finding in brain research, however. And this is precisely what seems only to work when the consciousness is switched off. So that is why we need to sleep. 

presented by hkk Krankenkasse


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