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.

There are three major characteristics that need to be exhibited for something to be defined as sleep:

• A regularly occurring period of inactivity.
• During that period, the animal needs to respond slower to a stimulus — sort of like drowsiness when humans first wake up.
• Finally, the sleep needs to be necessary to the animal's survival.

What they did: The researchers designed a series of experiments to test each of these criteria.

• Reduced activity: They observed the jellyfish for 6 days, and found that they pulsed 32% more slowly at night.
• Stimuli response: As their name suggests, upside-down jellyfish spend most of their time upside-down on the sea floor. When the researchers moved them to the top of the tank, they quickly returned to the bottom. But if they were dropped into the column at night, they took longer to re-orient.
• Skipping sleep: How do you keep a jellyfish awake? Squirt it with water, apparently. The scientists scheduled a pump that sent a pulse of water towards the jellyfish every 20 minutes. After a constant night of harassment, they were 17% less active the next day.

What's next: Jellyfish sleep, but do they dream? What does their sleep look like neurologically? Do the same genes that regulate sleep in humans regulate them in jellyfish? The researchers hope others will look into these questions.

One last thought: When we think about sleep research, we usually think about brain scans and REM cycles. But this study suggests that at its core, a nightly slumber is fundamental and necessary. "If sleep is found in such a simple and basic animal, maybe the function is simple and basic as well," says Michael Abrams, another author on the study. Though of course, he notes, jellyfish aren't actually simple. They've been evolving just as long as humans have.