Do Jellyfish Really Sleep?

Even if we don’t fully understand all the mechanics of sleep, we know that it is an important physiological process that helps all creatures to achieve rest and relaxation. And perhaps we don’t quite fully understand sleep as well as we previously believed. This is just one conclusion that scientists have reached after conducting a new study on jellyfish.

“We just went in one night, in the middle of the night, with our iPhones and some pretty simple apparatuses and we filmed them,” explains Michael Abrams. He is one of three leading authors behind this new study published in the journal Current Biology. “You could really clearly, by eye, see a change in the behavior between the light and the dark.”

This “change in behavior” he is talking about has to do with what we had previously associated with the biological definition of sleep. The biological definition actually says there are three criteria for “sleep”: reduced [physical] activity, reduced awareness of stimuli, and homeostatic regulation. The former two, basically, need a kind of quick shut down switch that flips the power off rather quickly so you can rest and then restore. The latter, though, is just a scientific way of saying that when you don’t sleep a lot for a while, your body will require more sleep later (and you’ll try to catch up).

So what does this have to do with jellyfish?

Well, the Cassiopea jellyfish is among the oldest—and most primitive—living species, so observing their sleep patterns could give us far more insight on the mechanics of this physiological need. And what the scientists found, “in the middle of the night” was that Cassiopea jellyfish exhibit the first criteria for sleep.

You see, this species pulse their bells during the day to create currents around their bodies. The team observed this pulsing is reduced at night.

So they devised to test the second criteria of sleep: awareness. Applying several other variables, the team attempted to wake the jellyfish up at night. They [mostly] succeeded; or, rather, they concluded that jellyfish do sleep, but not quite the same way as we do.

Which is fine, honestly, because humans are the most complex organisms on the planet, and jellyfish don’t even have a brain!

Of course, California Institute of Technology biologist Ravi Nath puts it a little more eloquently: “These results suggest that even those animals that lack a centralized nervous system require sleep, which means that sleep is one of the most ancient behavioral states, deeply rooted within the animal lineage.”

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