You would think so, but Americans have shown that you don’t need a brain to sleep. Paul Sternberg, a biologist at the California Institute of Technology (Caltech) in Pasadena believes that he has shown Cassiopea jellyfish do in fact sleep.
Among other tests, they built special aquariums where cameras monitored the pulsing of 23 jellyfish for almost a week. At night, the jellyfish slowed to 39 pulses per minute. Whereas the pulse rate was about 60 per minute during the day. To see whether they were asleep when pulsing slowly, they lifted the animals off their preferred place on the bottom of the tanks to the surface and then measured how quickly the jellyfish swam back to the bottom. The jellyfish tested at night were slow to respond, possibly indicating some sleep inertia. They then lifted them again 30 seconds later, and this time, the Jellyfish swam immediately to the bottom.
While suggestive of sleeping, without a more detailed understanding of what is happening in the nervous system it is hard to confirm. Other mechanisms may actually explain this behaviour.
Originals articles can be found here:
https://vanwinkles.com/how-do-jellyfish-sleep
http://www.sciencemag.org/news/2017/09/you-don-t-need-brain-sleep-just-ask-jellyfish
