What an ancient jellyfish can teach us about the evoution of sleep

Cassiopea jellyfish seem to have a sleep state despite the fact they don't have a brain. THAIFINN/Shutterstock

An upside-down jellyfish drifts in a shallow lagoon, rhythmically contracting its translucent bell. By night that beat drops from roughly 36 pulses a minute to nearer 30, and the animal slips into a state that, despite its lack of a brain, resembles sleep.

Field cameras show it even takes a brief siesta around noon, to “catch up” after a disturbed night.

A new Nature Communications...