When Ravi Nath asks people if jellyfish sleep, he finds that everyone thinks they know the answer. Roughly half say yes, and half say no. Some scientists assert that only mammals and birds could be said to truly sleep. Other people think that even plants have something akin to sleep. “Every person we’ve asked has an opinion,” Nath says. “Even a 10-year-old kid has a response.”

Nath has an answer, too. Along with his friends and fellow California Institute of Technology students Claire Bedbrook and Michael Abrams, he put a jellyfish called Cassiopea through a gauntlet of clever experiments, which confirmed that they do indeed enter a sleeplike state. Every night, they become less active and less responsive. They can be easily roused from this state, but if they’re deprived of their slow periods for too long, they become even more inactive and unresponsive the next day—as if they were reeling from an all-nighter. And if the trio are right, their discovery has big implications for understanding both how sleep evolved—and why.

Sleep is widespread across the animal kingdom. Fish sleep. Flies sleep. Even nematode worms, which Nath studies, sleep. But jellyfish belong to one of the most ancient animal groups, which split off from those other creatures at least 600 million years ago. If they also have a version of sleep, it suggests that the roots of this behavior are more ancient than anyone suspected.

It’s still unclear why exactly animals sleep at all, but there’s no shortage of explanations. Scientists have variously argued that sleep helps individuals to flush toxins from their brains, to consolidate new memories, or to reset their brains for a fresh day of learning. But none of these hypotheses make much sense for a jellyfish because they don’t have brains at all. They just have a nerve net—a loose ring of neurons that runs around the rim of their pulsating bells. “Maybe the drive for this sleep state was something more basic, like conserving energy,” says Bedbrook. “It might be something that’s required if you have a nervous system, regardless of how simple or complex it is.”

She and her friends started studying Cassiopea in their own apartment, eyeballing the movements of their bells by the light of their iPhones. Cassiopea is an upside-down jellyfish. It rarely swims, and instead sits inverted on a surface, using its pulsating bell to waft water over its upward-pointing tentacles. And these pulsing movements became slower at night. “You can really tell when you look at them at night that they’re less active than they are during the day,” says Bedbrook.

Rather than relying on their eyes, the trio designed an imaging system that would automatically count the jellyfish’s pulses over several days and nights. “The second we had the setup working, we could see the pattern right away,” Bedbrook says. The jellyfish are 30 percent less active at night. They pulse less frequently, and they go through several pauses, 10 to 20 seconds long, where they stop pulsing altogether.

But sleep isn’t just about inactivity. It’s defined by several other criteria—and the trio started checking off every one.

First, the inactivity must be reversible—if you can’t wake up, you’re more comatose than asleep. And indeed, when the trio roused their jellyfish by offering them a night-time snack, they saw that the animals became just as active as they’d normally be by day.

Second, a sleeping animal should be unresponsive—it should take more effort to rouse them when they’re asleep than when they’re awake. To test for that, the trio would place their jellyfish in a PVC pipe with a screen bottom. By lifting the pipe and then dropping it, they could briefly suspend the animal in mid-water. Cassiopea doesn’t like floating freely, and will typically swim down to a surface. But at night, they were much slower to do so; it’s as if they were groggy after having just woken up.

“This was when things became very convincing to me,” says Bedbrook. “Sometimes when we dropped the jellyfish in the water at night, they wouldn’t pulse. They’d just float to the bottom of the tank. It’s so different to how they respond to the stimulus during the day.” But if they repeated the experiment just 30 seconds later, the jellyfish responded as they would do during the day. “Now they’re awake!” says Abrams.

Finally, a sleeping animal should suffer if it’s deprived of sleep, and rebound the night after. “If we pull an all-nighter, we’re really tired afterwards,” explains Rath. He and his friends duplicated this effect in their jellyfish by blasting them with jets of water every 20 minutes. After each blast, the animals started pulsing more quickly before dropping back to their inactive state. And after a full night of this horrendous snooze alarm, they were less active the following day. By contrast, the same water jets, delivered during the day when the animals are typically awake, had no such effect.

That’s an impressive set of results. But for Nadine Gravett from the University of Witwatersrand, whose work on sleeping elephants I’ve covered before, it’s not enough. She says that scientists typically study sleep by strapping electrodes to the heads of animals and measuring the electrical activity of their brains. Without that, “they can’t claim that jellyfish have a sleep-like state,” Gravett says. “The most that can be said is that under controlled laboratory conditions, they show distinct periods of immobility—and it’s possible that sleep could happen during these periods.”

But Isabella Capellini from the University of Hull notes that electrical recordings are only used to quantify sleep in mammals and birds, and not in, say, fish or insects. “When studying sleep in such animals, we can only rely on the behavioral definition,” she says.

And based on that definition, the Caltech trio’s experiments “strongly argue for the existence of sleep in jellyfish,” adds Cheryl Van Buskirk from California State University. “It suggests that sleep may not be a product of nervous-system complexity, but rather may be a basic property of neurons. Why excitable cells would require periods of dampened activity is still a mystery, but it wouldn’t surprise me if the answer were to come from studies, like this, of simple nervous systems.”

Nath gets the skepticism. It’s the same reaction that researchers got when they first started claiming the flies or worms can sleep—something that’s now more accepted. But he’s not saying that jellyfish sleep is exactly the same as human sleep. It’s something more ancient and ancestral. It has many of the same core features but, as an example, there’s no obvious analog of REM sleep—the period where our closed eyes flicker rapidly, our muscles relax, and our brains conjure vivid dreams. Do jellyfish dream of gelatinous sheep? No one can say.

The team’s next move is to look at the jellyfish’s genes. The genes that influence sleep in flies and worms are the same as those that affect the slumbers of mice and humans—and based on a preliminary glance at Cassiopea’s genome, they seem to exist there too. “We’d have to block them in some way to see how that affects their behavior,” says Abrams.

Certainly, the jellyfish seems to respond to the same sleep-inducing chemicals that work on human brains. When the trio dosed their animals with melatonin, a hormone that makes people sleepy, they started pulsing more slowly. And when they treated the jellyfish with an antihistamine—a class of drugs that often makes people drowsy—they also started pulsing more slowly.

“We did try caffeine and we didn’t see anything though,” says Bedbrook.

“So not everything works,” adds Abrams. “We wanted to say something about giving coffee to jellies, but it didn’t work out.”