Mammalian animals with brains sleep to store memories and clear metabolic waste from the brain. It is akin to shutting down a computer to update it. But jellyfish, which do not have brains, also sleep for eight hours a day. When they did not sleep properly, they also became drowsy.
Lior Appelbaum, a professor in the Department of Life Sciences at Bar-Ilan University in Israel, and colleagues said in the journal Nature Communications on the 7th that "the core function of sleep evolved hundreds of millions of years ago in jellyfish and sea anemones, the first organisms with nervous systems." It means human sleep first appeared in the ocean.
◇Jellyfish sleep at night, sea anemones are daytime nappers
Jellyfish and sea anemones belong to cnidarians. The name comes from the fact that they use a stinging cell called a nematocyst when prey touches their bodies. Cnidarians have nerve cells arranged in a relatively simple network throughout the body and are classified as the first animals in which a neural network evolved.
Appelbaum's team observed Cassiopea andromeda, a jellyfish that lives upside down for life, and Nematostella vectensis, a starlet sea anemone, in tanks with cameras for 24 hours. Jellyfish generally have tentacles extending below a hemispherical body like an open umbrella, but Cassiopea floats with its tentacles facing upward.
For half of the day, the researchers shone light to distinguish day and night. Both jellyfish and sea anemones slept eight hours a day. Sleeping one-third of the day is similar to humans. However, jellyfish sleep at night, while sea anemones mainly napped during the day. Jellyfish also took short daytime naps of one to two hours.
During lit hours, jellyfish contracted and expanded their umbrella-shaped bodies more than 37 times per minute. They responded quickly to light or food, indicating wakefulness. At night, without light, the frequency of body movements decreased and it took longer to respond to light or food. They fell asleep. It took jellyfish 20 seconds to respond to stimuli at night, more than twice as long as during the day.
The fact that jellyfish sleep was first confirmed in 2017 by researchers at the California Institute of Technology in the United States. The researchers said, "It was already known that Cassiopea andromeda sleeps at night, but the precise sleep pattern had been unclear until now," and added, "It is also the first time we have confirmed that sea anemones sleep."
◇Like humans, they prevent DNA damage while sleeping
Making up for lost sleep the next day was also similar to humans. To disrupt sleep, the researchers agitated the water for six hours at night. The next day, the jellyfish slept 50% longer than peers that had rested sufficiently. When melatonin was injected into the tanks, jellyfish and sea anemones fell asleep even during their usual active periods. Melatonin is a hormone secreted at night in the human brain that promotes sleep.
Sleep is a very dangerous state for animals. When sleeping, they cannot tell if predators are approaching and lose time that could be used to find food or mates and care for offspring. The researchers explained that the purpose of sleep in jellyfish is the same as in humans. Even without a brain, they have a simple neural network that needs time for maintenance.
Analysis showed that DNA damage accumulates in neurons when jellyfish are awake, but such damage decreases when they sleep. As DNA damage increased, repair time also increased. When ultraviolet light was applied to increase DNA damage in jellyfish neurons, they slept longer. When a chemotherapy agent that causes DNA damage was administered to sea anemones, they slept 30% longer the next day than other peers.
Scientists believe neurons first appeared in early multicellular animals hundreds of millions of years ago. These animals resembled today's jellyfish or sea anemones, with neurons connected to each other but no central command center like a brain. If so, the core function of sleep to prevent neuronal damage can be seen as having evolved hundreds of millions of years ago.
Appelbaum said studying the evolution of sleep could also help clarify the link between sleep deprivation and neurodegenerative diseases. He added that it could also explain phenomena such as "local sleep," in which parts of the brain become temporarily inactive while awake. Appelbaum said, "We plan to study animals such as sponges, which do not have nervous systems, and zebrafish, whose core brain features are similar to those of humans."
References
Nature Communications(2026), DOI: https://doi.org/10.1038/s41467-025-67400-5
Current Biology(2017), DOI: https://doi.org/10.1038/10.1016/j.cub.2017.08.014