Author: Timothy Hearn, Lecturer, University of Cambridge; Anglia Ruskin University

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 […]