It turns out even brainless creatures such as jellyfish need to sleep. This extraordinary discovery, reported on by researchers at the California Institute of Technology, Pasadena, makes sleep even more mysterious than it already is.

Cassiopea jellyfish are the only animals we know of that sleep despite lacking a centralized nervous system. Credit: Caltech

Cassiopea jellyfish are the only animals we know of that sleep despite lacking a centralized nervous system. Credit: Caltech

When you’re tired as heck and finally hit the sack, there aren’t many things that can disturb your slumber. That’s the case for me, at least, much to the despair of my fellow ZME staff writers who have to snooze my alarm clocks while I continue to drool on the couch. When we do, however, have to forgo sleep, that little thinking box up your shoulders goes out of order — until you eventually tap shut down. 

This oh so familiar pattern has the obvious implication that sleep and higher nervous functions are deeply connected, with the former replenishing cognition. It follows that you need a sort of brain to sleep in the first place. Or so we used to think.

The brainless sleeper

Paul Sternberg, a biologist at Caltech, along with colleagues, wanted to see just how little brain an animal has to have to need sleep. They lowered the bar to no brain at all by studying several jellyfish species from the genus Cassiopea. These particular jellyfish hang motionless in shallow waters with their tentacles facing upward towards the water’s surface. To feed and sweep away waste, the animals pulse their bells about once per second.

Inside aquariums, the biologists studied 23 jellyfish with special motion-sensing cameras that snooped day and night for almost a week. During the night, the animals clearly slowed their movement to only 39 pulses per minutes compared to the typical 60 per minute at day.

Where these slow-pulsers asleep? The Caltech grad students lifted some of the jellyfish from their resting place at the bottom of the aquarium towards the surface and measured how quickly they reacted. During the night, jellyfish were far slower to respond by moving back to the bottom of the tank, just like a person is groggy and sluggish after being abruptly woken up.

At night, Cassiopea jellies pulse less frequently. This may be a clue that the animals are sleeping. Credit: Caltech.

At night, Cassiopea jellies pulse less frequently. This may be a clue that the animals are sleeping. Credit: Caltech.

The team even went a step further by sending pulsing water across the tank at night every 20 minutes for 6 or 12 hours. They found the jellyfish were not nearly as active the next morning as their peers who were left to their own devices. When the bothered jellyfish were finally left off the hook the following night, they seemed to have recovered by the next day. Again, this is parallel to how animals with brains would react if sleep deprived.

Interestingly, when the researchers sprinkled some food into the tank, the jellies became active again.

“It’s like the odor of coffee permeating your consciousness in the morning,” Sternberg says in a statement. 

Finally, the biologists gave the jellyfish melatonin, which is the hormone associated with sleep onset and a common drug which people take to doze off faster. The substance knocked the jellyfish out, the team reported in the journal Current Biology, with massive implications for sleep research.

“It’s important,” Sternberg said, “because it’s [an organism] with what we think of as a more primitive nervous system. … It raises the possibility of an early evolved fundamental process.”

While jellyfish don’t have a brain, they do have a ring-shaped nervous system embedded inside their bell-shaped bodies. These most recent findings suggest that nerve cells or nerve clusters require time off as well. Even more interestingly, jellyfish, which are positioned very early on the tree of life, could hint that sleep is as old as life itself.

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