The need for sleep is common to all the creatures in the world, from jellyfish, fish to humans. Theoretically, there is no evolutionary need in sleep, because sleeping animals are vulnerable to danger.
Especially to prey by enemies, and the biological reason that long sleeplessness leads to death has always been a mystery.
This reinforces the argument that sleep is essential, and that its rejection causes brain systems to collapse.
A study by Bar Ilan University, published in Nature Communications Journal, offers a groundbreaking theory of sleep vitality: When we sleep, our neurons ‟relax‟ from their normal functioning and allocate resources to reduce the DNA damage that has accumulated during wakefulness.
The research was carried out in the lab of Prof. Lior Appelbaum from the Faculty of Life Sciences and the Interdisciplinary Center for Brain Research, led by Appelbaum's doctoral student, David Zada.
What distinguishes the new theory is that it assumes that sleep is important at the level of the individual cell and not just at the laval of whole animal.
Since all the animals that have been sleeping and studied so far have nerve cells. And these cells have a nucleus, the researchers hypothesized that sleep is related to changes in the nucleus, where DNA is found.
However, the researchers admits that the results were surprised, and they did not realize how strong this correlation was.
Sleep and the Lonely Cell
The researchers used zebrafish — animals that in their youth are transparent, including their skull — allowing them to observe in real time what is happening in their brains.
Like humans, they also sleep at night (unlike other model animals like rats and mice who are night activists). To track the movement of chromosomes, which contain the DNA, the researchers developed a method for marking their neurons in high resolution so they can distinguish between individual cells.
The researchers photographed the nerve cell at different points of time and found that when the fish is awake, the movement of chromosomes in the nucleus is low and DNA damage is formed in the form of a double strand break.
If the fish remains awake for a long time, the damage can reach dangerous levels.
What was the damage?
According to the researchers, there can be several reasons: for example, ultraviolet radiation, which we are all exposed to at one level or another; The accumulation of free radicals that oxidize the DNA; Or simply a high activity of the nerve cells that requires the production of proteins that are needed for activity: In order to start production of proteins, RNA copies from the nucleus must be produced from DNA, and the formation of two-stranded fractures makes this process easier.
Whatever the cause of the damage, the movement of the chromosomes is necessary for its repair, as the damage has not been corrected effectively when it has been artificially stopped.
During sleep, the chromosome movement is about twice of waking, which is evidence of the efficient maintenance of the cell nucleus, where DNA is found.
In other words, the researchers argue that sleep returns the levels of damage to DNA to normal values that the cell can handle in each neuron.
But DNA maintenance is not efficient enough when neurons are awake and busy.
It is similar to a road with heavy traffic that accumulates damage during the day and can be maintained efficiently only at night when traffic is low.
Do other cells also correct their DNA when the animal is asleep?
The researchers looked at two other types of cells:
- Endothelium cells that are part of the blood vessels in the brain
- Glial cells, the supporting cells of the nervous system
The researchers found no difference in the chromosome dynamics between night and day in those two types of cells.
So at least now it seems that the mechanism is specific just to the neurons.
Tilt of Balance
Based on the findings, the researchers suggested a model that describes the role of sleep.
According to this model, there is a balance during the day between the creation of damage in the DNA and its repair.
The balance of the day tends to produce damage, because cell activity is high, while the movement of the chromosomes is too small to allow for effective repair of the damage.
When the damage accumulates beyond a certain threshold, networks of nerve cells start to trigger a sleep, and then the balance tends towards the correction of damages.
There are several experiments that reinforce this claim: for example, when you cause fish to sleep in the middle of its waking period by adding Melatonin (the sleep hormone) to the water, you see it in the growth of the chromosome movements.
On the other hand, disruption of the fish sleep during the night reduces the movement of the chromosomes to levels similar to natural waking during the day.
The next stage of the study will be to examine what happens in nerve cells that are specifically active during sleep in different areas of the brain.
Another question is what happens to the mechanisms of DNA repair in humans in general and during a dream in particular.
Another interesting direction to research is why there is a link between lack of sleep, DNA damage, and degenerative diseases.
For example, lack of sleep is correlated to increase the risk of Alzheimer's disease.
These studies are naturally more relevant to humans, but researchers are also interested in seeing what happens in other mammals, and even in non-mammalian animals such as worms and flies.
To concludes we can say: Sometimes we tend to give up sleep when we have something important to do and we think sleep is less important because anyway we will wake up in the morning.
The importance of this research, and sleep research in general, is that it shows that this is a misconception: good sleep is not a luxury but a need.