Jerusalem, 7 January, 2026 (TPS-IL) — Israeli researchers have discovered that sleep may have begun hundreds of millions of years ago in some of the earliest animals, including jellyfish and sea anemones, and that its primary purpose has remained the same ever since: protecting nerve cells from damage. The study suggests that improving sleep quality may slow the progression of neurodegenerative diseases like Alzheimer’s and Parkinson’s.
Sleep is widespread among animals with nervous systems, but it comes with serious risks. While asleep, animals are less aware of their surroundings, leaving them vulnerable to predators and interrupting essential behaviors like feeding. “This has always puzzled scientists: why would evolution preserve such a risky behavior?” said Prof. Lior Appelbaum, head of Bar-Ilan’s Molecular Neuroscience Lab. The new research suggests the answer lies in the essential need to repair neurons, a benefit so important that it outweighed these dangers from the very beginning.
Earlier research from Appelbaum’s lab showed that in zebrafish, neurons accumulate DNA damage while awake and require sleep to repair it. Neurons are particularly vulnerable because, unlike most cells, they do not divide and cannot easily be replaced.
In the current study, led by Dr. Raphael Aguillon and Dr. Amir Harduf, researchers observed sleep in two ancient species: diurnal jellyfish that are active during the day and sleep at night, and crepuscular sea anemones that rest in the morning. Using infrared video tracking and behavioral analysis, the researchers found that both species sleep roughly eight hours per day, similar to humans.
The research revealed that nerve cells in both species accumulate DNA damage while awake and repair it during sleep. When animals were kept awake, they slept longer afterward — a phenomenon known as “sleep rebound.” Increasing DNA damage, whether through UV light or chemical exposure, triggered additional recovery sleep, while promoting sleep with the hormone melatonin reduced DNA damage.
“These findings show that sleep and cell health are tightly linked,” the researchers said. “Damage increases the need for sleep, and sleep actively repairs it.”
The study also highlighted differences in sleep regulation. Jellyfish primarily rely on the light-dark cycle, while sea anemones are guided by an internal body clock. Despite these differences, both species depend on sleep to maintain neuron health.
“Our findings suggest that the ability of sleep to protect neurons is an ancient trait already present in some of the simplest animals with nervous systems,” Appelbaum said. “Sleep may have originally evolved to give neurons a dedicated period for repair, a function so fundamental that it has persisted across the animal kingdom.”
The research has clear implications for humans. Sleep problems are linked to memory loss and a higher risk of neurodegenerative diseases like Alzheimer’s and Parkinson’s, which may involve ongoing nerve cell damage. Researchers see potential for developing therapies that enhance the brain’s natural repair processes, either by promoting restorative sleep or by mimicking its protective effects.
“Sleep is important not just for learning and memory, but also for keeping our neurons healthy,” Appelbaum said. “The drive to protect nerve cells that we see in jellyfish and sea anemones may help explain why sleep is essential for humans today. This discovery gives us an evolutionary reason to take sleep seriously and reinforces its role in long-term brain health.”
The study was published in the peer-reviewed Nature Communications.
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