A primitive type of jellyfish called Cassiopea, which goes to sleep nightly, is seen on the floor of their tank at Caltech in Pasadena, California, U.S. in this image released on September 20, 2017. Courtesy Caltech/Handout via REUTERS

Scientists said on Thursday they have demonstrated that a primitive type of jellyfish called Cassiopea goes to sleep nightly. While sleep has been confirmed in other invertebrates such as worms and fruit flies, the jellyfish is the most evolutionarily ancient animal that has been shown to slumber.

“These results suggest that even those animals that lack a centralized nervous system require sleep, which means that sleep is one of the most ancient behavioral states, deeply rooted within the animal lineage,” California Institute of Technology biologist Ravi Nath said.

Jellyfish have thrived in the seas for at least 600 million years, longer than nearly any other animal. By comparison, dinosaurs appeared roughly 230 million years ago and humans appeared roughly 300,000 years ago. The findings involving such a primordial creature raise fresh questions about sleep’s origin and purpose.

“We do not know if sleep is limited to just animals,” said Nath, who helped lead the study published in the journal Current Biology.

“Sleep is a genetically encoded behavioral state. Genes and neural circuits interact to generate the sleep state,” Nath added. “I think it would be hard to demonstrate a sleep state in an organism that is not an animal, but I think the sleep state that we know may have been co-opted from periods of quiescence in organisms as diverse as plants, bacteria and fungi.”

Jellyfish are among the first animals to have developed neurons - nerve cells - though they lack a brain, spine or central nervous system.

Cassiopea jellyfish live in clear, shallow, tropical waters of the Pacific and western Atlantic oceans, eating plankton. Measuring about 1-2 inches (2.5-5 cm) in diameter, they are dubbed the “upside-down jellyfish” because they lie on the seafloor inverted in the water with their tentacles upward.

Through lab experiments, the researchers determined Cassiopea met three important sleep criteria: periods of decreased activity known as behavioral quiescence; a decreased response to stimuli; and an increased sleep drive after being sleep deprived.

The jellyfish were found to display periods of inactivity at night, pulsing their bodies 30 percent less often than during daytime. When a platform underneath them was removed, they took up to 5 seconds to “wake up” and reorient themselves. And when deprived of nighttime sleep by being nudged with a squirt of water, they became more likely to sleep during the day.

The researchers did not examine whether jellyfish dream.