Taken together, these data expand our understanding of the phylogenetic extent of sleep and reinforce the idea that sleep evolved early in the evolutionary history of animals. However, additional studies on other types of animals are required for a comprehensive understanding of the origin(s) and evolution of sleep.

What is the 'simplest' animal that sleeps? When did sleep first evolve? Do all animals sleep? Tantalizing hints to answers come from new research showing that jellyfish, one of the earliest evolving groups of animals, have a sleep-like restful state.