One of the big, unsolved mysteries in science is exactly why humans and other animals sleep. There are plenty of hypotheses: it’s believed sleep is important in letting our organs repair themselves and is key in developing memories. But it’s unclear why that much-needed maintenance only happens after we fall into an unconscious state, making us vulnerable to predators. One way of solving the riddle of sleep is to investigate its origins, and a new study in the journal Nature involving zebrafish reveals that the pattern of sleep found in humans and many other animals evolved at least 450 million years ago.
Researchers built a special fluorescent light-sheet microscope that could image the entire tiny fish down to single cells. They also genetically engineered fish so their muscles and neurons would light up in the presence of calcium, which is released when those cells are active. They then immobilized sleeping two-week-old zebrafish, which are transparent, in an agar solution. They were also able to capture the heart rate, eye movement, muscle tone and other data using a fluorescence-based sleep study apparatus they developed.
They found that the fish went through sleep cycles similar to humans, including periods the team dubbed “slow burst sleep” and “propagating wave sleep.” While they did not exhibit random eye movement (REM), which is ubiquitous in mammals, their eyes did roll back in their sockets. Their brain and muscle signatures, however, as well as the hormones that regulate sleep were all similar to those found in mammals. “They lose muscle tone, their heartbeat drops, they don't respond to stimuli—the only real difference is a lack of rapid eye movement during REM sleep,” senior author Philippe Mourrain of Stanford University says in a press release.
The study suggests that the basics of sleep emerged before mammals and fish diverged from an ocean-dwelling common ancestor 450 million years, about 150 million years earlier than previously believed. “These signatures [of sleep] really have important functions — even though we may not know what they are — that have survived hundreds of millions of years of evolution,” the study’s first author Louis Leung, a sleep researcher at Stanford, tells Tina Hesman Saey at Science News.
“We truly did not expect to find so many similarities with human and mammalian sleep,” Mourrain tells Kashmira Gander at Newsweek. “To see, in a live vertebrate, the complex choreography of brain and muscle activity during wake-sleep transitions and sleep was mind-blowing.”
The finding means that researchers may be able to use the little fish—already a staple in the science labs—to look into sleep disorders and to test sleep drugs. Currently, many studies rely on mice, which are nocturnal, to investigate sleep. Leung says in the press release that the fish might be a better stand-in for humans. “Because the fish neural signatures are in essence the same as ours, we can use information about them to generate new leads for drug trials,” he says. “As zebrafish are diurnal like humans, it’s perhaps more biologically accurate to compare fish sleep with humans’ for some aspects.”
While the new technology used to image the sleeping fish is being widely praised, Jenny Howard at National Geographic reports that not everyone thinks the sleep cycles in the fish are analogous to mammals. For one thing, sleep scientist Jerry Siegel at the University of California, Los Angeles, points out that the researchers looked at very young fish, and that sleep patterns in juveniles and adults are almost universally different in the animal kingdom.
“You can’t just say sleep is sleep,” he says, pointing out that among mammals sleep patterns are incredibly varied with some species getting three hours of shut-eye per day and others drifting off for 20 hours. Some animals have REM cycles, and some do not.
Mourrain, however, is more optimistic about the little swimmer's use in sleep research, even if the zebrafish’s nightly routines isn’t exactly the same as humans. “People forget that vertebrates are all very similar in their body organization and organs,” he tells Newsweek’s Gander. “We not only share a backbone protecting our spinal cord, the rest of the brain and neurochemistry allowing neurons to communicate is extremely conserved.”
He points out that the fish are already used in lots of medical research, including cardiovascular and cancer research, and believes this study shows they could also be used in sleep and neuroscience studies as well.