Researchers at the Stanford University School of Medicine have found that neural signatures in sleeping zebrafish are analogous to those of humans, suggesting that the brain activity evolved at least 450 million years ago, before any creatures crawled out of the ocean.
Scientists have known for more than 100 years that fish enter a sleep-like state, but until now they didn’t know if their sleep resembled that of land animals.
The researchers found that when zebrafish sleep, they can display two states that are similar to those found in mammals, reptiles and birds: slow-wave sleep and paradoxical, or rapid eye movement, sleep. The discovery marks the first time these brain patterns have been recorded in fish.
“This moves the evolution of neural signatures of sleep back quite a few years,” said postdoctoral scholar Louis Leung, PhD.
Philippe Mourrain, PhD, associate professor of psychiatry and behavioral sciences, is the senior author of the research. Leung is the lead author.
To study the zebrafish, common aquarium dwellers also known as danios, the researchers built a benchtop fluorescent light-sheet microscope capable of full-fish-body imaging with single-cell resolution. They recorded brain activity while the fish slept in an agar solution that immobilized them. They also observed the heart rate, eye movement and muscle tone of the sleeping fish using a fluorescence-based polysomnography that they developed.
They named the sleep states they observed “slow-bursting sleep,” which is analogous to slow-wave sleep, and “propagating-wave sleep,” analogous to REM sleep. Though the fish don’t move their eyes during REM sleep and don’t have eyelids, but the brain and muscle signatures are similar.
The researchers found another similarity between fish and human sleep. By genetically disrupting the function of melanin-concentrating hormone, a peptide that governs the sleep-wake cycle, and observing neural expressions as the fish slept, the researchers determined that the hormone’s signaling regulates the fish’s propagating wave sleep the way it regulates REM sleep in mammals.
Other aspects of their sleep state are similar to those of land vertebrates, Mourrain said: The fish remain still, their muscles relax, their cardio-respiratory rhythms slow down and they fail to react when they’re approached.
“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,” Mourrain said, though he added, “The rapid movement of the eyes is not a good criterion of this state, and we prefer to call it paradoxical sleep, as the brain looks awake while one is asleep.”
While scientists can’t say for certain that all animals sleep, it appears to be a universal need among vertebrates and invertebrates. Animals will die if they are deprived of sleep long enough, and people who fail to receive adequate sleep suffer from mental problems such as memory lapses and impaired judgment, along with a higher risk of disorders such as obesity and high blood pressure.
The exact benefits of sleep are still a mystery, however. “It’s an essential function,” Mourrain said, “but we don’t know precisely what it does.”