Scientists just discovered the first brainless animal that sleeps

Snoozing jellyfish have confirmed that a brain is not necessary for sleep. Scientists made the discovery after observing a primitive jellyfish called Cassiopea that lives upside down on the sea floor and lacks any kind of central nervous system.

The creatures, which resemble miniature cauliflower heads, have bodies that pulse with a steady rhythm. Videos of the jellyfish taken over 24 hours showed that they pulsed less frequently at night, but they could be “woken up” by dropping a little food into the tanks where they were kept.

“It’s the first example of sleep in animals without a brain,” said US researcher Professor Paul Sternberg from the California Institute of Technology (Caltech). He compared the effect of the food to that of smelling coffee in the morning. A second sign of sleep was seen when the jellyfish were physically disturbed during their low-activity periods, forcing them to float in open water. At night it took them three times longer to recover and return to a resting position. Such “grogginess” was typical of sleeping animals, said the scientists.