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 sleeplike 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 sleeping 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.

A paper describing the research was published July 10 in Nature. Philippe Mourrain, PhD, associate professor of psychiatry and behavioral sciences, is the senior author. 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, the brain and muscle signatures are similar. (Fish also don’t close their eyes when they sleep, as they have no eyelids.)

Sleeping like the fish

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 relaxed, their cardio-respiratory rhythms slow down and they fail to react when they’re approached.