Researchers have discovered that jellyfish sleep for about eight hours a day, including midday naps, much like humans. This finding in brainless marine creatures suggests sleep's benefits may be ancient and widespread. The study highlights how sleep helps repair neuronal damage in these simple animals.
Scientists at Bar-Ilan University in Ramat Gan, Israel, led by Lior Appelbaum, observed the upside-down jellyfish Cassiopea andromeda to uncover its sleep habits. Using cameras, they recorded the jellyfish in tanks over 24 hours, simulating day and night with alternating light periods. During daylight, the jellyfish pulsed their bell-shaped bodies more than 37 times per minute and quickly reacted to stimuli like bright light or food, indicating wakefulness. At night, pulsing slowed, and responses delayed, signaling sleep.
These pulses aid in feeding and oxygen distribution, Appelbaum explained. Overall, the jellyfish slept roughly eight hours, primarily at night, with a 1- to 2-hour midday nap. Previous research confirmed nocturnal sleep, but this work details the full pattern. When researchers disrupted sleep by pulsing water at the jellyfish, the animals compensated by sleeping more the next day. “This is like us: if we’re sleep deprived during the night, we sleep during the day because we’re tired,” Appelbaum noted.
Further tests showed DNA damage building up in the jellyfish's neurons during wakefulness, which sleep mitigated. Exposing them to ultraviolet light increased damage and prompted longer sleep periods. The team observed similar effects in the starlet sea anemone (Nematostella vectensis), marking the first evidence of sleep in these cnidarians.
As one of the earliest-evolved animals, jellyfish insights could reveal sleep's origins. Sleep likely first appeared in such marine life, aiding neuron maintenance before complex brains developed. The findings appear in Nature Communications (DOI: 10.1038/s41467-025-67400-5). More studies are needed to check if these patterns hold in other species or mammals.