people want to sleep, as do other mammals, fish and insects, and researchers have even found evidence of sleep in nematodes. Now, a new study suggests that jellyfish may also need sleep.

if this conclusion is true, it will be an intriguing thing that will help people better understand how sleep evolves. Jellyfish are very closely related to us vertebrates, and they don't even have brains, so if they sleep, it means that sleep has evolved in animals a long time ago.

how do researchers know if jellyfish are sleeping? They studied inverted jellyfish (Cassiopea) and observed their behavior at different times of the day. The jellyfish usually have regular "pulsations" in their bodies, like this:

as a result, they did find a sleep-like period in jellyfish: during this period, the jellyfish's "pulse" slowed down significantly, although it could still wake up. But they also become slow to respond to external stimuli. The researchers also tried to disturb the jellyfish from resting, and as a result, the "not getting enough" jellyfish behaved like sleep-deprived people the next day: they became less energetic and less responsive to the outside world.

this result looks interesting, but it doesn't convince all peers. Other researchers have commented that unless neural activity is measured, it is too early to say that jellyfish sleep. However, judging from the current results, it can at least be said that the "resting state" of jellyfish is similar to the sleep of other animals. Next, the researchers hope to find and study genes associated with sleep regulation in jellyfish.

I think the most interesting thing is that in order to show that the "sleep" state of the jellyfish is indeed similar to the sleep as we know it, they also gave some medicine to the jellyfish. In their paper, they mentioned that two sleepy things-melatonin and antihistamines-can put jellyfish in a similar state. It is said that they also tried to caffeine the jellyfish, but no obvious phenomenon was observed

reference:

http://www.cell.com/current-biology/fulltext/S0960-9822(17)31023-0

https://www.theatlantic.com/science/archive/2017/09/even-jellyfish-sleep/540432/

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pictures from the original paper