An intriguing new study is offering insights into the origins of sleep, for the first time discovering zebrafish brains shift through several brain states while sleeping. The researchers suggest these states could be analogous to human slow-wave, and REM sleep states, making zebrafish a potential candidate for use in human sleep research.

Almost every animal on the planet with a brain is known to enter some kind of sleeplike state. In humans, of course, sleep consists of a number of different complex phases, characterized by shifts in neuronal signatures. We know mammals, birds and lizards demonstrate a similar degree of complexity in these shifting sleep phases but until now it has been unclear exactly what is going on in a fish brain when it transitions to slumber.

To try to solve this mystery, a team of scientists from Stanford University engineered a zebrafish so individual neurons would flash when activated. Then, they constructed a novel microscope that could examine a whole fish body with single-cell resolution. The fish were immobilized in a solution and then closely monitored as they entered what was known to be their unique sleep state.

Two novel brain activity patterns quickly stood our to the researchers as the fish transitioned through its sleeping period. One pattern is called "slow bursting sleep" by the researchers, and they suggest this pattern is analogous to slow-wave sleep in humans. The other pattern is labeled "propagating wave sleep", and this is suggested to resemble REM sleep in humans. Of course, fish don't exhibit the same rapid-eye-movement as humans in this well-known sleep phase, but other muscle and body signatures reportedly seen in the zebrafish during this phase are similar to that of a human REM phase.

While this fascinating discovery may not entirely prove zebrafish enter sleep phases that are exactly like humans', it does offer certain evidence that sleep behaviors in fish are much more complex than many realized. The study offers clear evidence that when a zebrafish enters a sleep-like state, it doesn't simply transition into a one-dimensional dormant period, but instead moves through a complex series of neural patterns that does resemble the sophistication of sleep in mammals.

From an evolutionary perspective the researchers say this points to complex brain patterns in sleep appearing at least 450 million years ago. The fact these neural signatures could be considered analogous to humans offers a hypothetical evolutionary throughline making complex sleep a much older proposition than ever previously considered.

"This moves the evolution of neural signatures of sleep back quite a few years," says lead author on the new study, Louis Leung.

Apart from this being a merely academic discovery, the researchers suggest there may be practical applications from this study as zebrafish could potentially be used in research as an effective animal model for sleep disturbances. Zebrafish are potentially easier and cheaper to utilize than mice, and offer a more relevant comparison to human sleep patterns.

"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," says Leung. "As zebrafish are diurnal like humans, it's perhaps more biologically accurate to compare fish sleep with humans for some aspects."

The new study was published in the journal Nature.