How sleep works

1. sleep homeostasis

In order to fall asleep, we need to be tired. This is regulated by sleep homeostasis - similar to hunger and satiety. The longer we are awake and the more active we are, the greater the need to sleep. It is assumed that, among other things, the increase in the molecule adenosine in the nerve cells ensures that sleep pressure gradually rises. During sleep, adenosine is broken down again and the sleep pressure drops until we wake up.

2. the inner clock

To ensure that sleep does not take place in bites, but rather in one piece, we have a clock, also known as an internal clock or circadian rhythm. This biological rhythm lasts around 24 hours, roughly the length of a day and a night, and ensures that all important bodily processes take place within this time frame - including sleep.

Darkness activates the release of the sleep hormone melatonin.

The internal clock is primarily influenced by the change between daylight and darkness. This in turn regulates the production of certain hormones and neurotransmitters. Humans are naturally diurnal creatures. When daylight reaches the brain via the retina of the eye, the production of the dark hormone melatonin in the pineal gland is inhibited, thereby promoting wakefulness. Darkness, on the other hand, activates the release of melatonin, which, together with the increase in adenosine, favours sleep.

3. the sleep cycle

Sleep itself also takes place in control loops, so-called sleep cycles. A sleep cycle lasts 70 to 120 minutes. We go through four to seven sleep cycles per night, each of which takes place in four different sleep stages. The first three sleep stages belong to the category of non-REM sleep and begin with the phase of falling asleep.

During the REM phase, we experience a particularly large number of intense dreams.

4th sleep phase

The sleep phase lasts 1 to 5 minutes, during which the muscles gradually relax and the brain waves slow down. However, sleep is still very superficial, more like dozing, and then passes into what is known as light sleep.

5. light or normal sleep

Over the next 10 to 60 minutes, the body temperature drops, the heart rate slows down and the muscles continue to relax. Sleep is considered stable, but it is still relatively easy to wake up in this phase. This changes with the transition to the so-called deep sleep phase.

6. deep sleep

The muscles are now very relaxed, blood pressure is low, breathing and heartbeat are very even, the eyes are very calm and the brain waves show particularly slow delta frequencies in the EEG (electroencephalography, a method of measuring the electrical activity of the brain). After around 20 to 40 minutes, deep sleep turns into so-called REM sleep.

7 REM sleep

REM is the abbreviation for «rapid eye movement». In this stage of sleep, we experience particularly frequent and intense dreams and the movement of the eyes under the closed eyelids is clearly recognisable from the outside. Breathing, pulse and brain activity are increased, while the movement of the muscles is largely paralysed so that the body does not physically act out the dream.

All sleep phases are equally important.

It is assumed that the processing of emotions takes place primarily in the REM phases, while deep sleep focuses on the consolidation of newly learnt information as well as regeneration processes and recovery. All sleep phases are equally important. As the night progresses, the length of the deep sleep phases decreases and that of the REM phases increases. Between the individual stages, we often wake up briefly and change our position, which we hardly remember after waking up.