“No live organism can continue for long to exist sanely under conditions of absolute reality,” wrote Shirley Jackson in the first line of her 1959 horror novel The Haunting of Hill House. “Even larks and katydids are supposed, by some, to dream.”
Jackson penned this line to introduce her haunted house, where the line between reality and dreams was perpetually blurred, but she was right: Most living animals, at least, absolutely must shut out reality for a period to stay functional—and alive.
Across the animal kingdom, “sleep” looks very different. Humans enter a state of dreamy unconsciousness. Dolphins shut off parts of their brain piecemeal. Great Frigate Birds sleep in seconds-long bursts as they soar through the air.
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Nevertheless, almost all animals sleep. In the past decade or so researchers have found that even those without a brain, such as jellyfish and hydras,—freshwater organisms with the simplest body plan of all cnidarians—do something that looks a whole lot like snoozing. So what was the first animal to snatch a nap?
There are no preserved sleeping dens with the fossils of ancient little creatures snoring in tandem of course. And though no one knows for sure, snoozing probably occurred more than half a billion years ago, and it was almost certainly done by a very simple creature. “My opinion is that when animals start having [a] nervous system that attached to muscle, they almost certainly slept,” says Michael Abrams, now a postdoctoral researcher at the University of California, Berkeley, who was a member of a team that discovered that jellyfish of the genus Cassiopea regularly enter a sleeplike state in 2017.
Scientists have yet to stamp a date on when this happened, but nervous systems go way back. One 2019 study suggests that the first organisms to develop neurons were small, flat, platelike creatures called dickinsoniids, which snacked on microbes during the Ediacaran period, between 635 million and 542 million years ago. While it’s not clear whether these were really the first animals with a nervous system, neurons likely evolved around this time, before the massive expansion in the number of species on Earth during the subsequent Cambrian period.
But there is still controversy about whether living things without a nervous system, such as sponges or even plants, could be said to sleep. “The nervous system may not be necessary to exhibit sleep; nobody knows,” says Taichi Itoh, chronobiology researcher at Kyushu University in Japan. “We are still looking for the origin of sleep.”
Every time you snuggle into your pillow and drift off to dreamland, you’re engaging in a biological necessity that promotes cell growth, consolidates memories, reduces inflammation and—when all is said and done—keeps you alive. But sleep is strange: Why would it behoove organisms to shut out the environment for hours and hours, making themselves vulnerable to predators and other dangers?
That’s a question that motivates researchers like Itoh and Abrams. Clearly, sleep is important for basic biological processes, and a growing body of research suggests that it promotes cell proliferation and growth, Abrams says. There’s a reason newborn babies sleep around 16 hours a day.
This state was once thought to be the territory of animals with a brain, but in 2017 Abrams and his colleagues reported their discovery that Cassiopea jellyfish seems to sleep in Current Biology. These jellyfish spend most of their time sitting bell-down on the ocean floor, pulsing regularly, a metronome that allowed the team to determine how active the jellyfish were. In the lab, the researchers tracked the jellyfish’s pulses over time and found that the cnidarians pulsed 32 percent less at night than during the day. If startled by having their tank floor dropped out from under them, the jellyfish exhibited a delayed response, but they would slowly “wake” if offered food. When denied their quiescent period with annoying pulses of water all night, the jellies’ own pulses became slower and longer the next day, and the creatures showed reduced responsiveness, just as if they were sleep-deprived.
Similarly, Itoh and his team reported in Science Advances in 2020 that tiny hydras of the species Hydra vulgaris also have a quiescent period that acts a lot like sleep. In the study, the hydras were nearly completely still during these periods. A light pulse could rouse them, but after about 20 minutes of quiescence, the animals responded slowly to such pulses, much like a groggy napper. When deprived of this sleeplike period with irritating vibrations that were applied to their tank, the hydras “slept” longer the next chance they got and fell asleep more quickly.
For researchers, these quiescent periods in jellyfish and hydras check all the boxes for sleep: reduced activity, which is nonetheless reversible in response to a disturbance; a delayed reaction to stimulus; a slowdown of behavior after deprivation of this quiet period; and a greater drive to rest more after losing out on quiescent time.
What’s more, similar genes and molecules seem to control sleep in cnidarians, as compared with other animals and humans. Give jellyfish melatonin, Abrams and his team found, and they’ll get sleepy—just as humans do. Melatonin also makes hydras snoozy, Itoh and his team found, and genes that control sleep in hydras are similar to those that do so in humans. Yet dopamine, which elicits wakefulness in people and other animals, makes hydras sleepy. This suggests that evolution plays around with these chemical pathways, altering them over the hundreds of millions of years since the hydra and human lineages diverged from each other on the evolutionary tree.
Cnidarians don’t have a brain nor any kind of central nervous system, but they do have neurons that connect through diffuse “nets.” A big question, Abrams and Itoh say, is whether animals without neurons sleep. The logical next place to look for sleep, Abrams says, is in sponges, which are animals without a nervous system that evolved at least 540 million years ago and possibly as early as 890 million years ago, according to a 2021 Nature study. It’s hard to grow sponges in the lab, however. And it is tricky to define “sleep” in an animal that doesn’t show movement: How can you tell if a sponge is resting?
Leaving the animal kingdom, plants do respond to the environment with movement, turning to face the sun as it marches across the sky. It might be possible to test if plants “sleep” by disturbing these rhythms, Abrams says, though it’s also a tricky experiment to set up. If plants could be said to sleep, he adds, their version probably looks very different than sleep in the animal kingdom. “It probably gets less informative to humans, but it does get to origins or evolution of sleep,” Abrams says. “What did the last common ancestor without a nervous system have that allowed sleep to pop up once you have a nervous system?”
The bottom line is that pinning down the first organism to take a nap involves pinning down the first organism to evolve neurons some half a billion years ago—probably a small, ocean-dwelling animal that took advantage of its nervous system to hunt for prey. On the more speculative front, sleep could be at least as old as sponges, which were among the first animals on Earth. But to this day, no one really knows if sponges snooze.