Birds do it, bees do it, even educated fleas do it: No, they don’t fall in love, they sleep. However, exactly why all animals with a nervous system evolved to sleep has been a longstanding scientific mystery. Slumber certainly feels great, but it doesn’t exactly make sense — why should we spend a third of our lives passed out?
In a study published Tuesday in Nature Communications, scientists say they’ve figured out why on the cellular level. The core cellular function of sleep, they explain, is to combat the neuronal DNA damage that accumulates during waking hours. Sleep allows neurons to perform the efficient DNA maintenance that’s essential to a healthy life: Scientists already know that less sleep means greater vulnerability to anxiety, frustration, and ill health, but now they’re closer to understanding exactly why that’s the case.
“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,” study lead Lior Appelbaum, Ph.D., said Tuesday. “Sleep gives an opportunity to reduce DNA damage accumulated in the brain during wakefulness.”
Applebaum and his team examined how sleep is linked to nuclear maintenance by examining one of the most frequently used model organisms for genetic and developmental studies: the zebrafish. These transparent zebrafish were genetically engineered so that the chromosomes in their neurons carried colorful chemical tags. While the fish were awake and asleep, the scientists observed the movement of DNA and nuclear proteins inside the fish with a high-resolution microscope, which can be seen in the video above.
They witnessed that when the fish were awake, the chromosomes were relatively inactive, and broken strands of DNA accumulated in the neurons. However, when the fish were asleep the chromosomes became more active, and the DNA damage that had accumulated began to be repaired. Subsequent analysis confirmed that in order to perform nuclear maintenance, single neurons need an animal to go to sleep.
The accumulation of DNA damage, says Appelbaum, is the “price of wakefulness.” During wakefulness, chromosomes are less active, leaving them vulnerable to DNA damage caused by radiation, oxidative stress, and neuronal activity. Sleep kickstarts chromosomal activity and synchronizes nuclear maintenance within individual neurons, allowing the brain to be repaired while it’s not being used to the extent that it is during the day.
“It’s like potholes in the road,” Applebaum says. “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.”
Anecdotally, we know that a good night’s sleep can be restorative. Now it appears that it’s quantifiably restorative for the brain as well, allowing it to naturally mend the damage of the day.
Sleep is essential to all animals with a nervous system. Nevertheless, the core cellular function of sleep is unknown, and there is no conserved molecular marker to define sleep across phylogeny. Time-lapse imaging of chromosomal markers in single cells of live zebrafish revealed that sleep increases chromosome dynamics in individual neurons but not in two other cell types. Manipulation of sleep, chromosome dynamics, neuronal activity, and DNA double-strand breaks (DSBs) showed that chromosome dynamics are low and the number of DSBs accumulates during wakefulness. In turn, sleep increases chromosome dynamics, which are necessary to reduce the amount of DSBs. These results establish chromosome dynamics as a potential marker to define single sleeping cells, and propose that the restorative function of sleep is nuclear maintenance.