A sleep-like state in Hydra unravels conserved sleep mechanisms during the evolutionary development of the central nervous system | Science Advances
https://advances.sciencemag.org/content/6/41/eabb9415Sleep Evolved Before Brains. Hydras Are Living Proof.
https://www.quantamagazine.org/sleep-evolved-before-brains-hydras-are-living-proof-20210518/