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Even Without a Brain, Jellyfish Still Need to Sleep

These simple, ancient creatures show just how deeply rooted sleep may be in the animal kingdom

Cassiopea jellyfish resting "upside-down" in their tank (Caltech)
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Sleep is a fundamental part of life for many organisms on Earth, and new research suggests that even jellyfish require a few Z's—a discovery that sheds light on just how old and widespread the practice of sleeping must be.

In a study published this week in the journal Current Biology, Caltech researchers recorded the first example of a organism that enters a sleep-like state that has a "nerve net"—a simple nervous system—and no centralized brain, reports Steph Yin for the New York Times.

There's a lot still unknown about the reasons and mechanics behind sleep, but scientists largely agree that it's a complex process that somehow involves memory, reports Sarah Kaplan for the Washington Post. Thus is seems pretty obvious that the brainless Cassiopea jellyfish wouldn't sleep. But a group of graduate students decided to put that assumption to the test.

Cassiopea are commonly referred to as upside-down jellyfish because they rest on the bottoms of their containers with their tentacles floating upward into the air—opposite to the position most floating jellyfish assume. The researchers observed how, when the lights were turned off in the room holding the jellyfish's habitats, the creatures appeared to "pulse" less frequently, and would stay still for long periods of time in some form of sleep-like extended rest.

To test if this was actually "sleep," the scientists studied a group of 23 upside-down jellyfish using a three-pronged approach over the course of a week, reports Yin. They were able to show that the jellyfish could be "awoken" from their slumber by poking them and feeding them in the middle of the night. And when the creatures appear to be sleeping they reacted less quickly to being moved or overturned than when they were awake.

Most intriguingly, just like you need a certain amount of shut eye, the researchers found the jellyfish also need their rest. When the upside-down jellyfish were kept in containers that had water pulsed through them three times an hour to disrupt their peace and quiet, they reacted the next day like most people do at the office after a bad night's sleep. They were 17 percent less active than normal in this sleep-deprived state, reports Carrie Arnold for Nature.

Other relatively simple and brainless animals sleep too, such as worms, notes Andrew Masterson for Cosmo. But jellyfish are the most ancient creatures on the evolutionary tree that we know requires its sleepy time. As Yin writes, jellyfish split off the evolutionary tree from the ancestors of most other living creatures hundreds of millions of years ago. This means that sleep could be a very, very old behavior at the root of nearly all animals, and not something that evolved later in more advanced organisms.

This insight could help researchers dig more into the "paradox of sleep," as co-author Ravi Nath described it to Kaplan.

“Sleep is this period where animals are not doing the things that benefit from a natural selection perspective,” Nath tells Kaplan. What exactly makes sleep so important that it would have evolved so early on among animals and still be around today isn't clear, but the researchers hope this study brings scientists closer to finding an answer.

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