Liked Sleep Evolved Before Brains. Hydras Are Living Proof by Veronique Greenwood (WIRED)

This frustrated Irene Tobler, a sleep physiologist working at the University of Zurich in the late 1970s, who had begun to study the behavior of cockroaches, curious whether invertebrates like insects sleep as mammals do. Having read Piรฉron and others, Tobler knew that sleep could be defined behaviorally too.

She distilled a set of behavioral criteria to identify sleep without the EEG. A sleeping animal does not move around. It is harder to rouse than one thatโ€™s simply resting. It may take on a different pose than when awake, or it may seek out a specific location for sleep. Once awakened it behaves normally rather than sluggishly. And Tobler added a criterion of her own, drawn from her work with rats: A sleeping animal that has been disturbed will later sleep longer or more deeply than usual, a phenomenon called sleep homeostasis.