The purpose and evolutionary origins of sleep are among the biggest mysteries in neuroscience. Every complex animal, from the humblest fruit fly to the largest blue whale, sleeps — yet scientists can’t explain why any organism would leave itself vulnerable to predators, and unable to eat or mate, for a large portion of the day. Now, researchers have demonstrated for the first time that even an organism without a brain — a kind of jellyfish — shows sleep-like behaviour, suggesting that the origins of sleep are more primitive than thought.
Researchers observed that the rate at which Cassiopea jellyfish pulsed their bell decreased by one-third at night, and the animals were much slower to respond to external stimuli such as food or movement during that time. When deprived of their night-time rest, the jellies were less active the next day.
“Everyone we talk to has an opinion about whether or not jellyfish sleep. It really forces them to grapple with the question of what sleep is,” says Ravi Nath, the paper’s first author and a molecular geneticist at the California Institute of Technology (Caltech) in Pasadena. The study was published on 21 September in Current Biology1.
“This work provides compelling evidence for how early in evolution a sleep-like state evolved,” says Dion Dickman, a neuroscientist at the University of Southern California in Los Angeles.
Nath is studying sleep in the worm Caenorhabditis elegans, but whenever he presented his work at research conferences, other scientists scoffed at the idea that such a simple animal could sleep. The question got Nath thinking: how minimal can an animal’s nervous system get before the creature lacks the ability to sleep? Nath’s obsession soon infected his friends and fellow Caltech PhD students Michael Abrams and Claire Bedbrook. Abrams works on jellyfish, and he suggested that one of these creatures would be a suitable model organism, because jellies have neurons but no central nervous system. Instead, their neurons connect in a decentralized neural net.
Cassiopea jellyfish, in particular, caught the trio’s attention. Nicknamed the upside-down jellyfish because of its habit of sitting on the sea floor on its bell, with its tentacles waving upwards, Cassiopea rarely moves on its own. This made it easier for the researchers to design an automated system that used video to track the activity of the pulsing bell. To provide evidence of sleep-like behaviour in Cassiopea (or any other organism), the researchers needed to show a rapidly reversible period of decreased activity, or quiescence, with decreased responsiveness to stimuli. The behaviour also had to be driven by a need to sleep that increased the longer the jellyfish was awake, so that a day of reduced sleep would be followed by increased rest.