Scientists Found That Jellyfish Sleep 8 Hours a Night and Take Naps Too

Jellyfish sleep around eight hours a night, take short daytime naps, and compensate for lost rest, all without having a brain. These behaviors, revealed in a new study, suggest sleep may be far more ancient and widespread than previously thought.

Led by Lior Appelbaum of Bar-Ilan University, the research focused on the upside-down jellyfish Cassiopea andromeda. Scientists used continuous video monitoring in lab tanks to observe how the jellyfish behaved under controlled light cycles. What they found was striking: it follows a structured daily routine of activity and rest, much like mammals do.

A Structured Sleep Cycle In Jellyfish

The jellyfish in the study, published in Nature communications, spent about a third of the day in a low-activity state, mostly at night, pulsing their bell-shaped bodies more slowly and reacting more sluggishly to light or food.

During simulated daylight, Cassiopea andromeda pulsed more than 37 times per minute and responded rapidly to stimulation. At night, that rate dropped significantly, and reaction times slowed. The ancient sea creature also takes short midday naps lasting one to two hours, completing a sleep schedule that closely resembles that of humans.

When the researchers disrupted the jellyfish’s nighttime rest by pulsing water in their tanks, the marine animal responded by sleeping longer the next day, a classic sign of sleep rebound.

“It’s funny: just like humans, they spend about a third of their time asleep,” explained Lior Appelbaum, one of the co-authors of the study.

Sleep Tied To DNA Repair

Beyond behavior, the researchers found a potential biological function for sleep in jellyfish: limiting DNA damage. While awake, damage accumulates in their neurons. But sleep appears to reduce this effect, helping preserve the cells’ structure and function.

“The balance between DNA damage and repair is insufficient during wakefulness, and sleep provides a consolidated period for efficient cellular maintenance in individual neurons,” stated the team.

According to the team’s findings, when the jellyfish were exposed to ultraviolet light, which increases DNA damage, they responded by increasing their sleep duration. Appelbaum explained that this reinforces the idea that sleep helps maintain neural integrity, even in animals without brains.

Sea Anemones Caught Snoozing in New Study

To test whether this phenomenon was specific to jellyfish or common across similar species, the researchers repeated their experiments on the starlet sea anemone (Nematostella vectensis). As stated by the latest research, the results were nearly identical, confirming that these animals also sleep.

This marks the first documented evidence of sleep in sea anemones. Together with the jellyfish results, it suggests that sleep may be a shared trait among cnidarians, dating back hundreds of millions of years.