Sleep Evolved Before Brains. Hydras Are Living Proof. | Quanta Magazine

It appears that simple creatures — including, now, the brainless hydra — can sleep. And the intriguing implication of that finding is that sleep’s original role, buried billions of years back in life’s history, may have been very different from the standard human conception of it. If sleep does not require a brain, then it may be a profoundly broader phenomenon than we supposed.