Sleep, we've been told, helps our brain rest and get ready for the next day. But if you're an animal without a brain, like the primitive jellyfish Cassiopea, would you actually need to sleep?
The answer, it turns out, is still yes.
Researchers at Caltech chose these upside-down jellyfish because they're incredibly easy to monitor. They tend to stay on the floor of their tanks, pulsing about 58 times a minute to suck up food and remove waste. Any shifts in their behavior would be quickly noticeable.
After six days and six nights, the researchers observed a few changes in how the jellyfish behaved. First, at certain points, their pulses slowed down to 39 times a minute. Second, if the jellyfish were disturbed during this slower pulse period, say by being picked up from the floor of the tank and then released, they'd drift more slowly back to the bottom than if they were disturbed at other points. Finally, if the researchers "poked" the jellyfish with jets of water to keep them awake, the jellyfish would then shift to the slower rate of pulses at times that they normally had been observed to be active.
All of these observations point to jellyfish entering a sort of sleep cycle, but to what end? There's no brain, after all. It's possible the jellyfish's nerves need rest instead. One additional conclusion that the study suggests is that sleep is something that evolved very early on (Cassiopea are very old creatures) and has not changed much over the span of history.
If you'd like to hear the researchers discuss their study, and take in some absolutely hypnotic views of Cassiopea swimming in a tank, the video below is for you.