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.