An remarkable new study has determined that jellyfish actually sleep, which would make them the first known animal without a brain or central nervous system to do so. Sleep is thought of as an activity that restores brains and gives us time to process memories, so we assume brainless animals like jellyfish have no need to do it, but apparently that’s not the case.

It’s a huge discovery because it means that sleep was development much earlier in evolutionary history than scientists had originally believed. The discovery was made after examining Cassiopea, which is a stationary jellyfish that lives in mangrove swamps and shallow waters.

These tiny creatures look more like a head of cauliflower than a jelly fish, but they are considered part of the same family. Scientists noticed that these animals were pulsing less frequently at night, prompting them to create an image processing program to record their pulses for six days.

“Researchers spotted a second sign of sleep after dropping the floor out from dozing jellies,” reads a statement from the Howard Hughes Medical Institute. “The team placed Cassiopea inside a PVC pipe with a mesh bottom and then dipped the pipe into the tank – so submerged jellies rested on the mesh instead of the tank floor. Then the researchers lowered the pipe deeper into the tank, forcing the jellies to lift off the mesh and float in open water. They don’t like that, Sternberg explains. During the day, Cassiopea will quickly pulse their bells and swim down to rest on the mesh again. But at night, it takes them about three times longer to start pulsing. It’s like the jellyfish are a little groggy, he says. This delayed response to stimulation is typical for sleeping animals.”