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—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.
For one, when we slumber, we are unresponsive to gentle nudging or soft sounds. “You have to poke an animal harder to get it to respond when it's asleep than when it's awake,” Joiner explains. But sleepers wake up immediately if they fall out of bed or hear a loud enough bang. Furthermore, if we cut our 40 winks short, we are sleepy the next day and typically sleep more soundly when we finally get to bed—so there is a need to get the right amount of slumber.
Fish, flies, and even worms that meet these criteria have a brain of sorts, where nerve cells cluster and may be capable of coordinating sleep. So three Caltech graduate students decided to see whether a simple jellyfish, which lacks a brain but sports a ring-shaped nervous system in its bell-shaped body, could also sleep. Ravi Nath, Claire Bedbrook, and Michael Abrams looked at several species of upside-down jellyfish in the genus Cassiopea, which tend to hang motionless in shallow tropical waters with its modified tentacles facing upward. Like corals but unlike most jellyfish, Cassiopea host algae in their bodies that help supply nutrients. They pulse their bells once per second, keeping water flowing over their bodies to sweep in food and sweep away waste.
To document whether these jellies sleep, the 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, 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.
But do jellyfish need sleep the way people do? To find out, the students deprived some of the animals of sleep by pulsing water across them every 20 minutes for 6 hours or 12 hours at night. The jellyfish were not as active the next morning—and were even less so if sleep deprived for 12 hours—but they completely recovered by the next day. As a final test, the researchers gave the jellyfish melatonin, an over-the-counter sleep-inducing substance, which seemed to knock out the jellyfish, they report. Thus, “We can tell they are sleeping in the same way you would tell that another person is sleeping,” Raizen says.
Anders Garm, a neuroscientist at the University of Copenhagen, is unconvinced: “I would hesitate to call it sleep until you actually look at what happens in the nervous system,” he explains. “There could be other mechanisms explaining this behavior.” For example, light may be controlling the variation in pulsing activity.
Others, however, agree with the Caltech team. “The authors do a good job of demonstrating that jellyfish fulfill the most fundamental criteria for sleep,” Joiner says. “These data strongly argue for the existence of sleep in Cassiopea,” adds Cheryl Van Buskirk, a geneticist who studies sleep at California State University in Northridge.
What’s more, the findings suggest that it’s not just brains that need sleep; individual nerve cells may crave it as well, Van Buskirk suggests. “It may be an inherent requirement of excitable cells.”
The next challenge will be to see whether sponges, which are even lower on the tree of life than jellyfish, sleep, too.