Do Jellyfish Sleep?

by spotmydive

A surprising discovery was brought to light by three young researchers from California Institute of Technology. One day, while having a cafe, they wondered if jellyfish could sleep like humans?

Humans are not the only creatures on earth capable of sleeping: mammals, fish and some insects like flies need rest. Sleep has very important functions in the animal kingdom because it allows to rest the central nervous system and to conserve its energy. Until now, sleep was considered a purely cerebral exercise that required a brain. Cassiopea jellyfish have only a small diffuse network of nerve cells distributed over their all bodies. Yet study shows that these specie of jellyfish also need sleep!

Cassiopea, a small jellyfish

This species of jellyfish lives in shallow tropical Pacific and western Atlantic waters. The specimens measure only 2.5 centimeters in diameter and have the curious peculiarity of evolving with the tentacles upwards.

Extensive experiments

To be considered in a state of sleep, experiments must validate three important criteria:


Periods of decreased activity known as behavioral quiescence.

The test: They quickly noticed that the jellyfish were experiencing nocturnal inactivity with 30% less energy to propel themselves than during the day.


A decrease in response to stimuli.

The test: Installed on removable platforms, jellyfish sometimes took five seconds to wake up and reorient themselves when the platform was removed.


An increase in sleep after being deprived of sleep.

The test: deprived of sleep during the night, jellyfish were more likely to sleep during the day with a noticeable decrease in activity.

The different experiments of our three young researchers on jellyfish have confirmed their theory. This discovery is extraordinary in itself because it highlights the first case of sleep in animals without brain. This could lead to thinking that all creatures on our planet have a hereditary “need” for sleep. Jellyfish colonized the seas during million of years, more than any other known creatures. These results raise new questions about the origin and purpose of sleep.