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The upside-down jellyfish Cassiopea andromeda sleeps for roughly 8 hours a day.Credit: Shahar Shabtai\/Alamy<\/p>\n
Neither jellyfish nor sea anemones have brains. But these animals sleep in ways strikingly similar to humans, according to a study published today in Nature Communications1<\/a>. The findings bolster a theory that sleep evolved, at least in part, to protect the DNA in individual nerve cells, helping to repair damage that builds up while animals are awake.<\/p>\n \u201cNeurons are very precious,\u201d says study co-author Lior Appelbaum, a molecular neuroscientist at Bar-Ilan University in Ramat Gan, Israel. \u201cThey don\u2019t divide, so you need to keep them intact.\u201d<\/p>\n Previous work has shown that jellyfish enter a sleep-like state, but this study is the first to characterize the phenomenon in sea anemones, and the first to describe both creatures\u2019 sleep patterns in detail. \u201cEvery time somebody adds to the list of species that sleep, it is a very important step for the field,\u201d adds Chiara Cirelli, a sleep researcher at the University of Wisconsin\u2013Madison.<\/p>\n Restorative rest<\/p>\n Sleep is a risky state for animals. It leaves them vulnerable to predators and environmental hazards, and it cuts into time that could otherwise be spent foraging, mating or caring for offspring. Scientists broadly agree that sleep must serve a fundamental biological function, because evolution has preserved it across all animals with nervous systems studied so far. <\/p>\n The idea that sleep might actually predate the evolution of a centralized nervous system gained traction in 2017, when Ravi Nath, now a postdoctoral neuroscientist at Stanford University in California, and his colleagues demonstrated the sleep-like state in jellyfish2<\/a>. \u201cThere is good evidence that sleep emerged with neurons,\u201d he says. \u201cThere is likely a core function, but each species has also adapted sleep to meet its own needs.\u201d<\/p>\n
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