Some of Earth’s first animals might have just given scientists their first evidence that sleep is not what we think it is.
Jellyfish, which first arose about 700 million years ago, are great playgrounds for studying the evolution of sleep because of their age—and because they have a complex set of nerve cells but no brain.
Experts have always assumed that brains are pre-requisites for sleep, since sleep is so intimately connected with memory, learning, and other neurobiological processes. But sleep’s enduring value over many millennia has remained a mystery. Sleep makes an organism vulnerable to attack. Plus, it’s time spent doing nothing as opposed to eating and building one’s energy stores.
Here’s Sarah Kaplan, reporting for The Washington Post:
“We know it must be very important. Otherwise, we would just lose it,” Bedbrook said. If animals could evolve a way to live without sleep, surely they would have. But many experiments suggest that when creatures such as mice are deprived of sleep for too long, they die. Scientists have shown that animals as simple as the roundworm C. elegans, with a brain of just 302 neurons, need sleep to survive.
Cassiopea has no brain to speak of—just a diffuse “net” of nerve cells distributed across their small, squishy bodies. These jellyfish barely even behave like animals. Instead of mouths, they suck in food through pores in their tentacles. They also get energy via a symbiotic relationship with tiny photosynthetic organisms that live inside their cells.
To see if the jellyfish Cassiopea showed any signs of sleeping, a handful of Ph.D. students at the California Institute of Technology snuck into their lab at night (which wasn’t actually against the rules—they just didn’t want their advisers commenting on the work) tested the species for a few behaviors: reversible quiescence (inactivity without being paralyzed or in a coma), an increased arousal threshold (they have to be “woken up”), and homeostatically-regulated quiescence (meaning, the jellyfish have a biological drive to sleep). The students tested that last one by pulsing water at the jellyfish every 20 minutes for a whole night. The next day, the jellyfish slept more heavily and behaved more strangely compared to what’s normal for them. The students published their findings in the journal Current Biology.
Their discovery shows that sleep could be a more fundamental process than we ever knew—one that evolved very early on in the history of life on Earth, and one that plays an unclear role in most species. We know what sleep does , but we don’t yet know why it’s absolutely necessary and why many animals spend so much time doing it. The gelatinous little swimmer known as Cassiopea could help us find out.