Sleep in animals

Sleep is broadly considered a biological necessity in virtually all animals. The large majority of such taxa with documented sleep physiology are bilaterians, though there is increasing evidence of sleep or sleep-like states in non-bilaterians such as Cassiopea jellyfish and hydra (both cnidarians), and sponges. The various criteria which biologists use to define sleep states have been observed in all other animal phyla, often with profound variation in function.

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