Researchers from Bar-Ilan University have succeeded in deciphering the ancient sleep mechanism and reveal that sleep protects nerve cells.
Sleep is a universal phenomenon in the animal world, but also one of the greatest evolutionary puzzles. Why do animals sleep, even though during sleep they are more vulnerable to predators and give up valuable time for eating and reproduction? A new and groundbreaking study from Bar Ilan University, published in the journal Nature Communications, provides a surprising and profound answer: The basic and ancient function of sleep is to protect nerve cells from cellular stress and the accumulation of DNA fragments, and this function appeared hundreds of millions of years ago in jellyfish and sea anemones, among the first creatures with a nervous system.
The study was led by the research groups of Professor Lior Applebaum and Professor Oren Levy from Bar Ilan University, and carried out by Rafael Aguyon and Amir Hardof, research students in their laboratories. The researchers sought to trace the evolutionary roots of sleep and examined two particularly ancient species: jellyfish, which are active during the day and live in symbiosis with algae, and sea anemones, which are not symbiotic and are active mainly in the evening and night.
Using infrared video surveillance and precise behavioral analysis, it was found that both creatures sleep an average of about eight hours a day, a duration surprisingly similar to human sleep. Despite their different lifestyles and the different mechanisms that regulate sleep-wake cycles, a clear common pattern emerged: DNA damage in nerve cells accumulates during wakefulness, while during sleep this damage decreases.
When the animals were deprived of sleep, levels of damage increased, but they then slept longer to compensate. This phenomenon, known as sleep reversion, allowed for recovery and reduction of the accumulated damage. Furthermore, when the researchers induced DNA damage using ultraviolet radiation or DNA-damaging chemicals, a significant increase in the need for sleep was observed. In contrast, administration of melatonin, a hormone that induces sleep, led to a decrease in levels of damage.
These findings indicate a clear bidirectional relationship: DNA damage increases the need for sleep, and sleep itself allows for repair and reduction of damage. Thus, the study suggests that protecting neurons from cellular stress and daily DNA damage may be the central evolutionary driver of the development of sleep.
The study also demonstrates differences in how sleep is regulated between ancient species. In jellyfish, sleep cycles are primarily controlled by the light-dark cycle, while in sea anemones, sleep is primarily regulated by an internal biological clock. However, in both cases, sleep is needed and both species depend on sleep to reduce DNA damage, whether sleep cycles are dictated by the environment or by internal timing.
Professor Lior Appelbaum, from the Faculty of Life Sciences and the Multidisciplinary Center for Brain Research at Bar Ilan University, explains that "the ability of sleep to reduce DNA damage in nerve cells is an especially ancient evolutionary trait, present even in the simplest creatures with a nervous system. According to him, this may have been the original function of sleep, providing a concentrated period of time for the maintenance and preservation of neural tissues, even before more complex functions such as learning, memory, and dreaming developed."
The findings also have broad implications for human health. Sleep disturbances are linked to cognitive decline and an increased risk of neurological diseases such as Alzheimer's and Parkinson's, diseases in which persistent DNA damage is often found in nerve cells. The evolutionary evidence provided by this study strengthens the link between sleep quality and long-term brain resilience.
As Professor Appelbaum concludes: "Sleep is not only important for learning and memory, but also for maintaining the health of the nerve cells themselves. The fact that this mechanism already exists in jellyfish and sea anemones emphasizes that sleep is likely one of the oldest, most basic, and most vital behaviors to have evolved in the animal kingdom, and we are only beginning to understand the depth of its importance."
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A pair of mounted cannons from the workshop of Prof. Ahituv