It’s hard to tell when fish are asleep because they keep their eyes open. But snooze they do, and recent research shows that sleeping zebrafish and sleeping people share striking similarities.
In a study published July 10 in Nature, Stanford researchers found that when zebrafish sleep, their brains can display two states similar to those in sleeping mammals, reptiles and birds: slow-wave sleep and paradoxical sleep.
The discovery marks the first time these brain patterns have been recorded in fish.
“The only real difference is a lack of rapid eye movement during paradoxical sleep,” said Philippe Mourrain, PhD, an associate professor of psychiatry and behavioral sciences and senior author of the study.
The discovery means sleep research relevant to humans can be conducted on zebra-fish, which are easy to study, in part because they are transparent, breed quickly and are inexpensive to raise. And unlike mice, the typical stand-in for humans in sleep research, zebrafish are awake during the day and sleep at night, just like humans.
Most neurological disorders are linked to sleep abnormalities — and zebrafish studies could help understand the connection, said Mourrain.
“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,” added postdoctoral scholar Louis Leung, PhD, the study’s lead author.
To record the brain and body activity of sleeping zebrafish, Leung built a scanning machine with a mini-aquarium. He transferred the zebrafish, one at a time, into the aquarium holding not only water but gelatinous agarose, which immobilized the fish so they were perfectly positioned for scanning.
Once the fish were asleep, researchers recorded changes in their heart rates, eye movements and muscle tone. The research revealed additional similarities to sleeping land vertebrates: The fish remain still, their muscles relaxed; their cardio-respiratory rhythms slow down; and they fail to react when they’re approached.
Scientists aren’t certain all animals sleep, but it appears to be a universal need among vertebrates and invertebrates. The benefits of sleep are still a mystery, however. “It’s an essential function,” Mourrain said, “but we don’t know precisely what it does.