Recent research shows that jellyfish share surprising similarities with humans, including a sleep pattern of approximately eight hours a day, complemented by short naps. Understanding the sleep behaviors of these marine creatures can shed light on the evolutionary significance of sleep.

“Interestingly, like humans, jellyfish spend about a third of their time sleeping,” states Lior Appelbaum from Bar-Ilan University in Israel.

In animals with brains, such as mammals, sleep is crucial for memory consolidation and the elimination of metabolic waste. However, it remains unclear why sleep evolved in jellyfish, which belong to the brainless cnidarian group and possess neurons arranged in simple networks.

Appelbaum and his team utilized high-resolution cameras to observe Cassiopeia Andromeda, an upside-down jellyfish, in a controlled aquarium environment. The jellyfish were subjected to cycles of light and darkness to replicate natural conditions.

During the simulated daytime, the jellyfish exhibited an average pulse rate of over 37 times per minute, demonstrating responsiveness to sudden stimuli. In contrast, their pulse rate decreased at night, and they became less reactive, indicating a sleep state. These pulsations are vital for nutrient distribution and oxygen supply within the jellyfish’s body, as explained by Appelbaum.

Overall, jellyfish typically sleep for about eight hours each night, interspersed with brief naps lasting one to two hours. Prior studies had confirmed nocturnal sleep in C. Andromeda, but the intricacies of their sleep cycles were previously unknown.

In another experiment, researchers simulated sleep disruption by pulsating water against the jellyfish, which led to even better sleep the following day. “It mirrors human behavior: when sleep-deprived at night, we tend to feel more fatigued during the day,” notes Appelbaum.

Crucially, further examination indicated that sleep in C. Andromeda is associated with reduced DNA damage. Sleep likely protects neurons from deterioration that might occur during wakefulness, as corroborated by the observation that exposing jellyfish to ultraviolet light—thereby increasing DNA damage—resulted in improved sleep patterns.

Future studies are required to determine whether similar sleep benefits apply to other jellyfish species or even mammals. The researchers also found comparable results with starlet sea anemones (Nematostella vectensis), marking a significant step in confirming sleep in these organisms, according to Appelbaum.