Jellyfish sleep about as much as humans do – and nap like us too

Jellyfish seem to sleep about 8 hours a day, nap at midday, and nap more after a bad night's sleep – just like us. Sleep is thought to have first evolved in such sea creatures, and a better understanding of their precise sleep patterns may help explain why it evolved in the first place.

“It’s funny: like people, they spend about a third of their time sleeping,” says Lior Appelbaum and Bar-Ilan University in Ramat Gan, Israel.

In animals with brains, such as mammals, sleep is critical for things like storage of memories And clearing the brain of metabolic waste. But it was unclear why sleep evolved in jellyfish, which belong to a group of mindless animals called cnidarians that are thought to have first evolved neurons arranged in a relatively simple network throughout the body.

Appelbaum and his colleagues used cameras to record. Cassiopeia Andromeda, a type of inverted jellyfish, in tanks for 24 hours. The jellyfish, which usually sit with their tentacles up in the shallow seafloor, were exposed to light half the time to simulate day and night.

The team found that when simulating daylight K. Andromeda people pulsed their bell-shaped body an average of more than 37 times per minute and reacted quickly to sudden bright light or food, suggesting they were awake. In contrast, they pulsed less frequently at night and took longer to respond to light or food, indicating they were sleeping. This pulsation is thought to help animals feed and distribute oxygen throughout their bodies, Appelbaum says.

In total, the jellyfish slept for about 8 hours, mostly at night, with a short midday nap lasting about 1-2 hours. Previous studies have already shown that K. Andromeda sleeps at night, but until now his exact sleep patterns were unclear, he says.

In another experiment in which researchers poured water on jellyfish to disrupt their sleep, the animals slept more the next day. “It's like us: If we're sleep deprived at night, we sleep during the day because we're tired,” Appelbaum says.

Importantly, further analysis showed that DNA damage accumulates in K. Andromedaneurons while he's awake, but sleep appears to reduce this damage, which would otherwise lead to neuronal degradation and damage, he says. Supporting this idea, when the team used ultraviolet light to repair DNA damage, the jellyfish slept more.

Further research is needed to see if the same thing happens in other species of jellyfish or even mammals, but the researchers saw similar results when they repeated the experiments on the star anemone (Nematostella vectensis) “providing the first evidence that sea anemones sleep,” Appelbaum says.