Electroencephalography and sleep

It was only with the help of electroencephalography that researchers succeeded in defining the sleep stages known today. Above all, the different wave frequencies and other peculiarities such as sleep spindles or k-complexes help to differentiate. First, a normal sleep cycle is described.

If you close your eyes, alpha waves with low amplitude can be displayed in the EEG. These waves change during sleep. On the one hand, the frequency decreases, one speaks of theta waves.

Furthermore, an increase in the amplitude of individual waves can be observed. Basically it can be said that the deeper one sleeps, the frequency decreases continuously while the amplitude always increases. This suggests a high synchronicity of the nerve cells of the cerebrum during the sleep process.

Sleep stage I is only a few minutes long and has a low wake-up threshold, i.e. only a weak external stimulus is needed to wake up the person. Sleep stage I is followed by sleep stage II, which is somewhat longer at about 15 minutes and also has a higher wake-up threshold. In the electroencephalogram, theta waves are measurable with a higher amplitude compared to stage I.

In addition, specific k-complexes and sleep spindles appear, which are characteristic for stage II sleep. The sleep stage III with long-wave delta waves is followed by stage IV, which is characterized by delta waves with high amplitude. This sleep stage has the highest wake-up threshold and lasts between 20-40 minutes.Although consciousness is largely cut off from sensory impressions during deep sleep, very intense stimuli can still reach the brain and lead to awakening.

This fact is a great advantage, especially in dangerous situations, because it enables the person to react as quickly as possible. The sleep stages III and IV are also referred to as “slow-wave” or synchronized sleep, based on their characteristics in the electroencephalogram. During deep sleep, the parasympathetic nervous system dominates the body.

It stimulates digestion, inhibits breathing and slows down the heartbeat. This is useful because the body needs to recover during sleep and to provide energy for the waking state. After sleep stage IV, the remaining sleep stages will be reversed until there is a significant change in the EEG after stage I is reached.

Waves of the waking state (beta waves) are registered and the amplitude decreases strongly, although the wake-up threshold remains very high. This is called desynchronized sleep. It is mainly dominated by reactions of the sympathetic nervous system.

The blood circulation in the brain increases strongly, heartbeat and breathing rate increase. In addition, excitation of the penis or clitoris can occur. The skeletal muscles are flaccid, only the eye and respiratory muscles show a certain tone.

Since eye twitches and eye movements often occur during desynchronized sleep, it is also called “Rapid Eye Movement (REM)” sleep. In addition, it should be noted that people who wake up from REM sleep are more likely to remember dreams. Therefore, it is assumed that people dream predominantly during REM sleep.

In the first sleep cycle, REM sleep lasts about 10 minutes, but it gets a little longer with each cycle. Normally, a person goes through between 5 and 7 sleep cycles in one night. Towards the end of sleep, REM sleep can last up to 40 minutes. Often sleep ends with this phase, although the wake-up threshold is comparatively high.