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takyon writes:

A study of jellyfish has challenged the idea that animals need a brain to exhibit sleeping behavior:

We think of sleep as restoring our brains: a time to process memories, cleanse our cells of toxins, and prepare for a new day. But even animals that lack brains need to snooze. Biologists have discovered that, like people, jellyfish hit the hay and have the same trouble we do waking up. Because these creatures are very low on the animal family tree, the work suggests that the ability to sleep evolved quite early.

"Sleep was likely present in the very first animals on this planet," says David Raizen, a neuroscientist and sleep expert at the University of Pennsylvania who was not involved with the work. "The results of this study challenge certain commonly held beliefs," adds William Joiner, a neuroscientist at the University of California, San Diego, who was also not involved with the work. "For example, that sleep requires a centralized nervous system and related neural circuits across evolution." Evidence from one recent study even suggests that skeletal muscles may be involved [open, DOI: 10.7554/eLife.26557] [DX]—at least in mice.

One of the biggest challenges of studying sleep is defining what "sleep" means. "Half the people say everything sleeps, and half say only humans and mammals sleep," says study author Paul Sternberg, a biologist at the California Institute of Technology (Caltech) in Pasadena. He and others argue that that fish, flies, and even worms nod off regularly. Humans and other animals rest for many reasons, they point out, but sleep differs from simply taking a breather in key ways.

[...] To document whether these jellies sleep, [grad] students built special aquariums where cameras monitored the pulsing of 23 animals day and night for almost a week. At night, the jellyfish slowed to 39 pulses per minute, compared with about 60 per minute during the day [DOI: 10.1016/j.cub.2017.08.014] [DX], they report today in Current Biology. To see whether these slow-pulsers really were asleep, the students lifted the animals off their preferred place on the bottom of the tanks to the surface and measured how quickly the jellyfish started swimming back to the bottom. Like a groggy person early in the morning, the jellyfish tested at night were slow to respond. But they eventually roused themselves, and when they were lifted again 30 seconds later, they swam immediately to the bottom.

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