It’s hard to imagine two animals more different than a jellyfish, a pulsating brainless blob, and a human, a super-intelligent bipedal ape-thing. Despite our great differences, we have one unlikely similarity: sleep.
It turns out, brainless jellyfish enjoy a snooze just as much as other animals, as reported in a new study published in Current Biology. The fact that these creatures exhibit a sleep-like state shows that a brain is not necessary for sleeping. More profoundly, it could even hint at the ancient evolutionary origins of the mysterious behavior.
“It may not seem surprising that jellyfish sleep – after all, mammals sleep, and other invertebrates such as worms and fruit flies sleep," said study co-lead author Ravi Nath, a graduate student in Caltech's Sternberg laboratory. "But jellyfish are the most evolutionarily ancient animals known to sleep. This finding opens up many more questions: Is sleep the property of neurons? And perhaps a more far-fetched question: Do plants sleep?"
The researchers made this discovery by monitoring jellyfish in a tank. They found that the animals go through periods of inactivity at nighttime, pulsing less than 39 times per minute, compared to 58 times per minute during the day.
This showed a phase of decreased activity, which happens when other animals sleep, but the scientists also needed to show that this was a true sleep-like state. So, they decided to keep them up one night by “poking” them with a jet of water. They found that this meant the jellyfish were more likely to fall into a sleepy state of inactivity the next day because they were kept up the night before. For a bunch of strange floating blobs, that’s actually pretty cute.
Sleep has previously been observed in worms, flies, and zebrafish. However, this is the first time it’s been shown to occur in a creature without a central nervous system. The researchers wrote that it suggests “this behavioral state arose prior to the evolution of a centralized nervous system” and has since remained untouched by millennia of evolution. Perhaps then, sleep is a fundamental part of life as we know it. To find out, scientists need to understand the unknown genetic mechanisms that underlie sleep.
"Many animals have the same genes that govern sleep," said Michael Abrams, co-first author and a graduate student in Caltech's Goentoro laboratory. "Though it was beyond the scope of our project to measure gene expression in jellyfish, we tested the effects of compounds that in other animals are known to promote sleep, such as melatonin. We found that these compounds did affect jellyfish sleep in the predicted ways, suggesting that their underlying sleep mechanism is similar to those of other organisms – including humans."