Is sleep a waste of time? Besides the fact that people will die without sleeping a minimum amount of time, researchers at Bar-Ilan University (BIU) in Ramat Gan have found that the time you snooze clears out damage to the DNA in your cells that accumulates during the time you’re awake.
Most people spend about a third of their lives sleeping. Throughout the long process of evolution, sleep has remained universal and essential to all organisms with a nervous system. Even invertebrates such as flies, worms and jellyfish sleep, even though their “time off” could make them victims of predators.
But the reason why animals sleep has remained a mystery and is considered among the biggest unanswered questions in life sciences.
In a new study titled “Sleep increases chromosome dynamics to enable reduction of accumulating DNA damage in single neurons” that was just published in the prestigious journal Nature Communications, BIU researchers reveal a novel and surprising function of sleep. They believe it could explain how sleep and sleep disturbances affect brain performance, aging and various brain disorders.
Using three-dimensional, time-lapse imaging techniques, they worked on live zebrafish – a freshwater fish belonging to the minnow family and native to the southern Himalayas that is not only a popular aquarium fish but also an important and commonly used vertebrate model organism in scientific research.
The researchers were able to show, for the first time, that single neurons require sleep in order to perform nuclear maintenance. DNA in the genes can be harmed by many processes including radiation, oxidative stress and even neuronal activity, but repair systems within each cell correct this damage. The new study shows that during wakefulness, when “chromosome dynamics” are low, DNA damage consistently accumulates and can reach unsafe levels.
The role of sleep is to increase chromosome dynamics and normalize the levels of DNA damage in each single neuron. Apparently, this DNA maintenance process is not efficient enough during the online wakefulness period and requires an offline sleep period with reduced input to the brain for it to occur.
“It’s like potholes in the road,” explained Prof. Lior Appelbaum of BIU’s Mina and Everard Goodman Faculty of Life Sciences and the Gonda (Goldschmied) Multidisciplinary Brain Research Center, who led the study. “Roads accumulate wear and tear, especially during daytime rush hours, and it is most convenient and efficient to fix them at night, when there is light traffic.”
Appelbaum calls the accumulation of DNA damage the “price of wakefulness.” He and his doctoral student David Zada, first author of the study – as well as co-authors, Dr. Tali Lerer-Goldshtein, Dr. Irina Bronshtein, and Prof. Yuval Garini – suggested that sleep consolidates and synchronizes nuclear maintenance within individual neurons; they decided to try to confirm their theory.
They managed to do it because they worked on zebrafish, whose bodies are absolutely transparent and that have a brain – believe it or not – that is similar to that of humans. According to zebrafish researchers at Brown University, this type of tropical fish has already proved useful for a huge range of neurological, developmental, behavioral and environmental studies by researchers in a wide range of disciplines.
Zebrafish, said the BIU scientists, are a perfect organism for studying a single cell in a live animal under physiological conditions. Using a high-resolution microscope to examine the fish, the team saw the movement of DNA and nuclear proteins within the cell while the fish were both awake and asleep. The researchers were especially surprised to find that chromosomes are more active at night, when the body rests – but this increased activity enables the efficiency of the repair to DNA damage.
The results established chromosome dynamics as a potential marker for defining single sleeping cells and propose that the restorative function of sleep is nuclear maintenance. “We’ve found a causal link [among] sleep, chromosome dynamics, neuronal activity and DNA damage and repair with direct physiological relevance to the entire organism,” concluded Appelbaum. “Sleep gives an opportunity to reduce DNA damage accumulated in the brain during wakefulness.”
“Despite the risk of reduced awareness to the environment, animals – ranging from jellyfish to zebrafish to humans – have to sleep to allow their neurons to perform efficient DNA maintenance. This is possibly the reason why sleep has evolved and is so conserved in the animal kingdom,” said Appelbaum.