We think of sleep as restoring our brains: a time to process memories, cleanse our cells of toxins, and prepare for a new day. But even animals that lack brains need to snooze. Biologists have discovered that, like people, jellyfish hit the hay and have the same trouble we do waking up. Because these creatures are very low on the animal family tree, the work suggests that the ability to sleep evolved quite early.

"Sleep was likely present in the very first animals on this planet," says David Raizen, a neuroscientist and sleep expert at the University of Pennsylvania who was not involved with the work. "The results of this study challenge certain commonly held beliefs," adds William Joiner, a neuroscientist at the University of California, San Diego, who was also not involved with the work. "For example, that sleep requires a centralized nervous system and related neural circuits across evolution." Evidence from one recent study even suggests that skeletal muscles may be involved [open, DOI: 10.7554/eLife.26557] [DX]—at least in mice.