Science reports:

You don’t need a brain to benefit from a good night of sleep. Despite lacking a central nervous system, jellyfish and sea anemones have sleep patterns remarkably similar to those of humans, researchers report today in Nature Communications.

The work supports the idea that sleep arose early in animal evolution to help the first neurons repair themselves, says Cheryl Van Buskirk, a geneticist at California State University, Northridge who was not involved with the research. “This study is another nail in the coffin of the idea that sleep evolved to manage complex, powerful brains.”

In nature, sleep is risky: Snoozing organisms are vulnerable to predators. Yet species across the animal kingdom spend multiple hours a day dozing off—even ancient groups including cnidarians, which include jellyfish, anemones, and corals—all among the earliest animals to develop neurons. Researchers have recorded sleeplike behavior in upside-down jellyfish in the genus Cassiopea and small freshwater relatives of jellyfish known as hydra. [Continue reading…]