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To some, Cassiopea jellyfish may appear constantly sleepy. They rarely swim, instead opting to lie on their backs on the seabed and gently pulse. But it hasn't been clear whether or not these "upside-down" jellyfish actually sleep — until now. A group of scientists from California recently made the surprising discovery that the listless creatures really do sleep at night. And, if they don't get enough sleep, they become groggy and sluggish the next day.

The findings are significant. They prove, for the first time, that animals without brains need sleep. The study, published in the journal Current Biology, reveals new possibilities about sleep’s evolutionary origins.

“We went for the simplest animal that we thought might sleep," said study co-author Ravi Nath, a graduate student at Caltech. "Going to jellyfish, we pushed this back to the root of sleep. It shows sleep must be rooted in an ancient and important function.” 

Academics have long been intrigued about which animals need to sleep. In some creatures, like humans and cats, the signs of sleep are unmistakeable. But as animals become more primitive, the necessity of sleep becomes more of a puzzle.

Scientists often study sleep in invertebrate fruit flies, and have found evidence that roundworms sleep, too. But these simple animals are still an evolutionary level above jellyfish, which have a "nerve net" of neurons spread around the body rather than a centralized nervous system.

“Humans and jellyfish are almost as distant evolutionarily as you can go," said co-author Claire Bedbrook, a graduate student at Caltech. "This really emphasizes how important the sleep state is, and gets you thinking about why something as simple as a jellyfish would even require sleep."   

Scientists from Australia had previously shown that deadly box jellyfish exhibit certain signs of sleep. But no one had demonstrated that jellyfish could fall into a full slumber. 

Biologically, sleep is a carefully defined term. To prove that the jellyfish were actually asleep — rather than simply tired or comatose — the scientists had to carry out a number of experiments.

First, they measured how frequently the jellyfish pulsed at night, compared to during the daytime. After analyzing their tank of 23 jellyfish for six days, they discovered that the pulse rate declined by around 32% at night.

Then, researchers had to find out if the jellyfish could be woken up easily — to prove they were experiencing sleep rather than paralysis or coma — which the researchers achieved by sprinkling a little food into their tanks.

Another element of the sleep test was finding out if the jellyfish became less responsive at night. 

They did this by measuring how long it took the sleepy jellyfish to move to the bottom of the tank, their favorite spot, at night compared to during the day. And, just like humans, the jellies were sluggish after being woken up. But they recovered quickly when scientists made them do the same task a second time.

The final task was to figure out whether the spineless study specimens displayed evidence of sleep deprivation. In what was surely a brutal night for the jellyfish, the researchers squirted them with water for ten seconds every 20 minutes. And, the next day, the sleep-deprived jellies were lethargic and inactive.

So what can we understand from a tank of sleepy jellyfish? Their behavior suggests that, at its essence, sleep might be a more basic process than scientists had previously realized. It's possible that the complex functions achieved by our eight hours in bed are merely the window dressing on a simple biological requirement.

“If sleep is found in such a basic and simple animal, it's likely the original function of sleep is also basic and simple," said Nath. "That means that the complicated sleep [features], like memory [consolidation] and sleep stages, were added on as decoration later on, as sleeping evolved." 

“It is pretty surprising that something as seemingly simple as a jellyfish has a sleep-like state,” said Dion Dickman, an assistant professor at the University of Southern California, who wasn’t involved in the study, but who researches sleep in flies.

The study, Dickman explained, supports the idea that sleep may have originally emerged as something distinct from the vital process for memory and learning that it's become in higher-order organisms.