Brainless jellyfish and sea anemones sleep to repair DNA, Bar-Ilan study finds

Why do animals sleep, when lying unconscious for hours leaves them exposed to predators? A study from Bar-Ilan University, published in Nature Communications on 6 January 2026, suggests the answer reaches back hundreds of millions of years to creatures without any brain at all. The work was led by neuroscientist Lior Appelbaum and marine biologist Oren Levy, with Raphaël Aguillon, Amir Harduf and colleagues.

The team studied two cnidarians, the upside-down jellyfish Cassiopea andromeda and the starlet sea anemone Nematostella vectensis. Both have a diffuse nerve net rather than a brain. Using infrared video tracking of animals raised in the lab, along with observations of jellyfish in their natural habitat off Eilat, the researchers found that both species sleep roughly eight hours a day, much like people, and that the longer they stay awake, the greater their need to sleep. The jellyfish's sleep follows the cycle of light and dark, while the anemone's is governed more by an internal clock.

The crucial finding concerned DNA. Damage to the genetic material in nerve cells built up while the animals were awake and fell while they slept. When the researchers kept animals awake, they later slept more, and that rebound sleep reduced the damage. Exposing them to ultraviolet light or a DNA-damaging chemical made them sleep more, while melatonin lowered the damage. The authors conclude that sleep likely first evolved to protect neurons from the wear and tear of daily activity, a function retained as more complex brains appeared.

Tel Aviv University sleep researcher Yuval Nir, who was not involved, told The Times of Israel the results point to an early evolutionary role for sleep in safeguarding the genome, with possible relevance to ageing and neurodegenerative disease. Appelbaum plans to look next at sponges, which have no neurons at all.