Why Do We Sleep at All? The Surprising Answer from the Sea
Researchers at Bar-Ilan University decode an ancient sleep mechanism and reveal that sleep protects nerve cells
Sleep is a universal phenomenon, yet it remains one of evolution’s greatest mysteries. Why do animals sleep, even though sleep makes them more vulnerable to predators and forces them to give up valuable time for feeding and reproduction? A new study from Bar-Ilan University, published in the journal Nature Communications, offers a surprising and profound answer: the most basic and ancient role of sleep is to protect nerve cells from cellular stress and the accumulation of DNA damage. This function appeared hundreds of millions of years ago in jellyfish and sea anemones, among the earliest organisms to possess a nervous system.
Tracing the Evolutionary Roots of Sleep
The study was led by the research groups of Prof. Lior Appelbaum and Prof. Oren Levy of Bar-Ilan University and carried out by Rafael Agyon and Amir Harduf, research students in their laboratories. The researchers set out to trace the evolutionary origins of sleep by examining two particularly ancient species: jellyfish that are active during the day and live in symbiosis with algae, and sea anemones that are non-symbiotic and active mainly during the evening and night.
Eight Hours of Sleep—Just Like Humans
Using infrared video tracking and precise behavioral analysis, the researchers found that both organisms sleep an average of about eight hours a day, a duration strikingly similar to human sleep. Despite their very different lifestyles and the distinct mechanisms regulating their sleep–wake cycles, a clear shared pattern emerged: during wakefulness, DNA damage accumulates in nerve cells, while during sleep, this damage decreases.
Sleep Deprivation, Recovery, and DNA Damage
When the animals were deprived of sleep, DNA damage levels rose. Subsequently, they slept for longer periods to compensate. This phenomenon, known as sleep rebound, enabled recovery and a reduction in the accumulated damage. Moreover, when the researchers deliberately induced DNA damage using ultraviolet radiation or DNA-damaging chemicals, a significant increase in the need for sleep was observed. In contrast, administering melatonin, a hormone that promotes sleep, led to a reduction in damage levels.
A Two-Way Link Between Sleep and Cellular Repair
These findings point to a clear bidirectional relationship: DNA damage increases the need for sleep, and sleep itself enables repair and reduction of that damage. The study therefore suggests that protecting nerve cells from cellular stress and everyday DNA damage may be the central evolutionary driver behind the development of sleep.
Different Regulators, Same Essential Need
The research also demonstrates differences in how sleep is regulated among these ancient species. In jellyfish, sleep cycles are governed mainly by the light–dark cycle, while in sea anemones sleep is regulated primarily by an internal biological clock. Nevertheless, in both cases, the need for sleep is fundamental, and both species depend on sleep to reduce DNA damage, whether sleep cycles are dictated by the environment or by internal timing mechanisms.
“An Exceptionally Ancient Evolutionary Trait”
Prof. Lior Appelbaum, from the Faculty of Life Sciences and the Gonda Multidisciplinary Brain Research Center at Bar-Ilan University, explains that “the ability of sleep to reduce DNA damage in nerve cells is an exceptionally ancient evolutionary trait, present even in the simplest organisms with a nervous system". According to him, this may have been sleep’s original function, providing a concentrated window for maintenance and preservation of neural tissue, "long before more complex functions such as learning, memory, and dreaming evolved.”
Implications for Human Health
The findings also have broad implications for human health. Sleep disorders are associated with cognitive decline and an increased risk of neurological diseases such as Alzheimer’s and Parkinson’s, conditions in which persistent DNA damage in nerve cells is often observed. The evolutionary evidence provided by this study strengthens the link between sleep quality and long-term brain resilience.
As Prof. Appelbaum concludes: “Sleep is not only important for learning and memory, but also for maintaining the health of nerve cells themselves. The fact that this mechanism already exists in jellyfish and sea anemones underscores that sleep is likely one of the most ancient, fundamental, and vital behaviors to evolve in the animal kingdom, and we are only beginning to understand the depth of its importance.”