Sleep and its brain functions

woman sleeping with sleeping mask

Dr. Jean-Yves Sovilla

FMH Specialist in Neurology, Centre de Médecine de l'Eveil et du sommeil (Medical Centre for Sleep-Wake Medicine), Clinique de Genolier, Genolier

August 15, 2022

Who doesn’t love a great night’s sleep, only to wake up feeling fully refreshed and energized. There are many interesting things going on with our brains while we sleep, many of them being discovered only recently. Biological processes that happen during sleep include storing new information, nerve cells reorganizing which helps the brain function, etc. We still don’t know everything that’s to know about sleep and brains functions during it, however, we do know it’s incredibly useful. The useful aspects are better concentration and focus, problem-solving skills, memory, creativity, decision making, and so on.

If you want to learn more, continue reading the article by Dr. Jean-Yves Sovilla.

For a long time, sleep was a black hole in the universe of knowledge, around which many vivid ideas were entwined. Hypnos (symbolising sleep) is the brother of Thanatos (symbolising death): sleep was considered a kind of temporary death. It was imagined that the spirit ceased, or even left the body while dreaming, or that the gods sent messages, etc.

By the 1950s, technology was then sophisticated enough to explain what was going on during sleep. Some of the hidden mechanisms were only discovered at all in the last 20 years.


One of the functions of sleep is the regulation of memory. The mechanism of anchoring in long-term memory is well known: during sleep, the synapses (connection points between neurons) are as if “strapped in” (potentiated for a long time) and make consolidation possible. A long series of experiments in humans and animals has shown that what is learned before a sleep phase is better stored than when learning during the day. And much better than in a test subject who is prevented from sleeping.

Studies in intracerebral microscopy in living animals show that synapses increase in size and number when awake and decrease in number during sleep. Thus they create a more efficient neural network, which does not happen in the awake animal. Even if there is no evidence for this yet, one can imagine that the typical intellectual performance reductions and physical clumsiness during sleep deprivation are related to the less efficient neural network.

Microscopic view of the same dendrite end (place where the axons connect to the neurons) in the living animal.


Recently, an important mechanism was discovered: throughout the body, an important part of the “cleaning” of the organs is carried out by the lymphatic system, except in the brain, which has no lymphatic vessels. Now, the brain has the greatest energy consumption in the body and produces a lot of waste that the bloodstream is not sufficient to dispose of. The brain floats in a fluid: cerebrospinal fluid (also called cerebrospinal fluid). This fluid has a complicated cycle from its formation in the ventricles (chambers of the brain) to its elimination by transfer to the veins that drain blood from the brain. The fluid enters the brain through extremely narrow, almost virtual, spaces (Virchow-Robin spaces). Now these spaces widen during sleep, with the brain swelling to act as drainage channels for larger waste – which cannot fit into the small blood vessels – to be carried away in the cerebrospinal fluid through the venous system and lymphatic system in the neck.


It is known that sleep deprivation inevitably leads to death, through metabolic catastrophe: despite unlimited access to food, water and at ideal temperature, the animal becomes powerless and dies.

A recent study shows an increase in average (arterial) blood pressure (risk of later hypertension) and glucose resistance (type 2 diabetes risk) when sleep is cut to 6 hours per 24 hours.

The hormonal system is completely under the control of the brain through the pituitary gland, which “directs” all the other glands, but this mechanism is intensely linked to sleep. Thus the sex hormones are secreted mainly at night, which women in the menopause notice by the hot flushes and men by the fact that they are “ready” in the morning.

These diagrams show the hormone levels depending on the waking or sleeping state of the test subject

A. Growth hormones are not secreted when the subject is not asleep. This hormone is not only necessary for growth, but above all for the regeneration of tired organs, through cell division.

B. Cortisol is sleep-dependent.

C. TSH (thyroid stimulating hormone, controls the thyroid gland) is increased during sleep deprivation (results in excessive energy consumption by the body).

D. Prolactin (stimulates milk production, but also has other effects, especially on libido) is blocked during sleep deprivation.

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