In a new study researchers at BarIlan University in Israel reveal an unexpected function of sleep that they believe could explain how sleep and sleep disturbances affect brain performance, aging and various brain disorders. Researchers used 3D time-lapse imaging techniques in live zebra fish and were able to define sleep in a single chromosome resolution and show, for the first time, that single neurons require sleep in order to perform nuclear maintenance. According to the current work, during wakefulness, when chromosome dynamics are low, DNA damage consistently accumulates and can reach unsafe levels. The study experts say that the role of sleep is to increase chromosome dynamics, and normalise 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 in order to occur. "It's like potholes in the road," said lead author Professor Lior Appelbaum, adding, "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 LererGoldshtein, Dr. Irina Bronshtein, and Prof. Yuval Garini, hypothesised that sleep consolidates and synchronises nuclear maintenance within individual neurons, and set out to confirm this theory. The scientists made the discovery thanks to the zebrafish model. With their absolute transparency, and a brain similar to humans, zebrafish are a perfect organism in which to study single cell within a live animal under physiological conditions. Using a high resolution microscope, the movement of DNA and nuclear proteins within the cell -- inside the fish -- can be observed while the fish are awake and asleep. The researchers were particularly 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 establish 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 between sleep, chromosome dynamics, neuronal activity, and DNA damage and repair with direct physiological relevance to the entire organism," said Professor Appelbaum adding that sleep gives people 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, and this is possibly the reason why sleep has evolved and is so conserved in the animal kingdom," he concluded.