Why do we sleep? Why do we invest the third part of our life in closing our eyes and disconnecting? Humans, like other animals with nervous systems, spend a good part of their lives sleeping. It is considered that sleep is a very active stage for the brain in which the toxins accumulated during the day are eliminated and that, without rest, this organ loses the capacity to create memories, concentrate or respond quickly. But apart from this, the biological function of sleep is a great mystery, and all in spite of the fact that it affects almost every type of tissue in the body, from the brain to the heart, and has an impact on various areas, such as the immune system and metabolism. That’s why it’s not surprising that chronic lack of sleep is related to hypertension, cardiovascular disease, diabetes, depression and obesity.
A study just published in the journal Nature Communications, by researchers at Bar-Ilan University (Israel), has revealed the existence of a new sleep function that affects the functioning of neurons. Through three-dimensional imaging techniques in zebrafish brains, scientists were able to observe the effects of sleep on chromosomes. Thus, they have discovered, for the first time, that individual neurons need sleep to carry out the maintenance tasks of their nuclei (the part of the cells where the DNA is found). As suggested, these observations could explain why lack of sleep can have effects on brain performance, aging, and various brain disorders.
In particular, observations have shown that, during wakefulness, genetic material accumulates damage to DNA, due to common phenomena such as radiation, the work of neurons themselves or oxidative stress (a phenomenon that occurs through the accumulation of free radicals from respiration, certain chemical reactions or the functioning of blood vessels). But during sleep, DNA maintenance and repair processes are activated and more efficient.
How to repair a road
“It happens like the potholes of a road,” said Lior Appelbaum, director of the investigation, in a statement. “Roads accumulate wear, especially during peak hours, and it is more convenient and efficient to repair them at night when there is little traffic.
The discovery has been made thanks to an animal perfect for observing the nervous system: the zebrafish. It not only has a brain comparable to the human brain in its structure, but it is also completely transparent. It is therefore very useful for observing cells individually, even in vivo.
Microscopic photograph of a zebrafish larva. In green, chromosome dynamics and in red of a single neuron
Microscopic photograph of a zebrafish larva. In green, chromosome dynamics and in red of a single neuron – David Zada
Through a high-resolution microscope they were able to observe the movement of the DNA and proteins of the nuclei, when the fish were awake and asleep. Thus they were able to observe, to their surprise, that the chromosomes are more active during the night, when the body rests, which increases the efficiency of the repair of the genetic material.
The importance of nuclear maintenance
“Sleeping gives the body an opportunity to reduce the damage accumulated in DNA during vigil,” said Appelbaum. “Despite the risk of reducing the level of alertness to the environment, animals have to sleep to allow their cells to maintain DNA efficiently, and this is possibly why sleep has evolved and is so conserved throughout the animal kingdom. According to this scientist, it is quite possible that the great and unknown function of sleep repair is precisely this: nuclear maintenance.
Indeed, the authors have confirmed that sleep consolidates and synchronizes nuclear maintenance in individual neurons. They have also established chromosome dynamics as a potential marker to define when a cell is sleeping or not. They have also created a causal link between sleep, chromosome dynamics, neuronal activity and DNA repair.