Jellyfish show sleep doesn't require a brain

Upside-down jellyfish. Credit: Peter Holderness / Caltech

Even jellyfish need their beauty sleep, according to a study conducted at CalTech and published Thursday in Current Biology. Jellyfish don't have brains, just a diffuse webbing of cells called a 'nerve net.' But despite their seemingly simple nervous system, they enter an inactive period each night. And when jellyfish don't catch enough Z's, they lag behind the next day — just like humans do.

Why it matters: It's (essentially) impossible to test if all animals sleep. But if "sleep is conserved from jellyfish all the way to humans, which are almost the furthest evolutionary distance you can go in animals," it would suggest sleep has ancient origins, Claire Bedbrook, one of the authors of the study, tells Axios. Not only does this mean sleep probably only evolved once long ago, it also "really highlights how important sleep is for animals," she adds.