Jellyfish and sea anemones, despite lacking brains, show sleep behavior similar to that seen in humans, according to a new study published in Nature Communications. Researchers found that these simple marine animals, like jellyfish, experience periods of rest that align closely with human-like sleep, offering new insight into how sleep functions in creatures with nervous systems but no centralized brain.
The study revealed that the upside-down jellyfish (Cassiopea andromeda) sleeps for about eight hours daily, primarily at night, with a short rest during the day. The starlet sea anemone (Nematostella vectensis), which was studied in a lab setting, also showed daily rest patterns for roughly a third of the day, mostly near dawn.
These observations were made both in controlled environments and in natural habitats, such as the waters near Key Largo, Florida.
Sleep function in brainless animals offers new clues
Lior Appelbaum, a molecular neuroscientist at Bar-Ilan University in Israel and co-author of the study, explained that neurons are delicate structures that do not regenerate through cell division.
He said preserving their integrity might be one reason sleep exists. The findings support the theory that sleep helps repair damage to DNA in nerve cells, which can accumulate during waking hours.
"Sleep is a conserved behavior across all animals with a nervous system, ranging from cnidarians to humans."
Read about the sleep #evolution.
A new study describes "a theory that sleep evolved, at least in part, to protect the DNA in individual nerve cells, helping to repair… pic.twitter.com/3MHVRaZlXH— Manuela Casasoli (@manuelacasasoli) January 7, 2026
Earlier research had confirmed that jellyfish enter a sleep-like state. However, this study is the first to document sleep behavior in sea anemones and to provide a detailed comparison of the two species’ rest patterns.
Chiara Cirelli, a sleep researcher at the University of Wisconsin–Madison, emphasized that identifying sleep in additional species adds valuable evidence to the field and strengthens the argument that sleep plays a fundamental role in biology.
Jellyfish and sea anemones show signs of human-like sleep
Although sleep exposes animals to risks like predators and environmental threats, scientists agree it must serve an essential purpose, as it has been preserved through evolution across a wide range of species.
Ravi Nath, a postdoctoral neuroscientist at Stanford University, noted that there is strong evidence linking the emergence of sleep to the development of neurons, and that different species have adapted the function to fit their unique needs.
By studying jellyfish and sea anemones, researchers continue to uncover how sleep evolved and why it remains vital even in the absence of a brain.
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