Every night as we plunge into unconsciousness, our bodies enter into a deeply rooted evolutionary act. As our eyes flit behind closed lids, we fall into four stages of sleep. So much is happening, and yet we feel like nothing is happening at all.
Sleep is a centerpiece to human biology, emerging and evolving to life on a spinning world that orbits the Sun. Without sleep, we die. Various forms of brain activity have been recorded in mammals, birds, and lizards as they slumber, but whether fish and amphibians display similar patterns to our own remained unclear.
Now, researchers from Stanford University claim they have detected neural activity patterns in the brains of sleeping zebrafish that are indeed akin to our own. This, they say, means the basics of this brain activity evolved before we ever even left the ocean for land.
"It's incredibly exciting to think that sleep and its functions are so deeply conserved," said neuroscientist and principal investigator Philippe Mourrain to IFLScience.
The discovery shifts the evolution of neural sleep signatures back at least 450 million years. Scientists have known for some time fish enter a sleep-like snooze, but they were unable to clarify to what degree their sleep mirrors that of mammals – and no wonder, it’s no easy feat to record the activity of a 1-millimeter-long brain.
"When we began to see activities in the zebrafish, we were stunned that while they were not identical, the signatures shared so many features with those seen in other animals," said neuroscientist Louis Leung, lead author of the study published in Nature. "In hindsight, we probably shouldn’t have been so surprised considering zebrafish, as a diurnal vertebrate, share 80-90 percent genes with us and the broad organization and neurochemistries between our brains is the same, but the existence of these neural signatures still had not been seen until now."
To peer into the fish’s micro-noggin, Leung and Gordon Wang of Mourrain’s lab built a new light-sheet microscope based on plans from an open source microscope platform (OpenSPIM) that could image to single-cell resolution. The fish slept immobilized in a jelly-like substance with their eye movement, muscle tone, and heart rate monitored.
"This microscope allows fast whole brain and whole body imaging of a fish that expresses a protein that flashes green light when its neuronal and muscular cells are activated. By recording the brain activity, the trunk muscle activity, the heartbeat, and the eye movement, we had invented polysomnography for fish," said Mourrain.
The team recorded slow-bursting and propagating-wave sleep, which they say is similar to slow-wave and REM sleep, respectively. Zebrafish, with no eyelids, did not display rapid eye movement like humans in REM. However, their muscles relaxed, their heart rate slowed, and they were less aware of their surroundings.
A few researchers not involved with the study have noted it’s difficult to draw a firm connection between sleep in zebrafish and mammals. The signatures recorded in juvenile fish also can’t be extrapolated to adult sleep. Still, the scanning technology is promising for future research.
The team can leverage the tech to more precisely screen for new drugs, including hypnotics and anesthetics. "This opens the door to the development and profiling of new sleep drugs with a much higher resolution throughout the brain and body that one could not visualize in an opaque mouse or human body," said Leung.
"As a take-home message, it is becoming more and more clear that sleep is an essential biology function not just for humans but for more and more animals that are studied. I encourage everyone to cherish what has taken hundreds of millions of years to create," said Leung.
