Sleep As Humans Experience It May Have Emerged 450 Million Years Ago

Scientists who studied a small, transparent fish believe the way humans sleep could have evolved 450 million years ago.

When mammals, along with birds and lizards, sleep, scientists think they experience what is known as deep sleep and rapid eye movement. For a study published in the journal Nature, this group of scientists wanted to find out more about how fish sleep.

Philippe Mourrain, study co-author and associate professor at Stanford University School of Medicine, told Newsweek the team set out to uncover whether sleep, as it is known in humans, is a more ancestral biological phenomenon than previously thought.

The zebrafish is a useful animal for scientists to study as it shares 84 percent of the genes that cause disease in humans. It also has a brain, heart, kidneys and liver, reports National Geographic. They are also diurnal like humans, unlike the rodents that are commonly used in research. Helpfully for senior author Mourrain and his colleagues, the zebrafish's brain is translucent.

The team studied zebrafish in a lab, kept in water that immobilized them. As the fish slept, the scientists used equipment to document their brain activity as well as their eye movement, heart rate and muscle dynamics.

"This method, which we call fluorescent polysomnography [fPSG], is completely non-invasive and doesn't require any surgery, it simply leverages the optical transparency of the fish," explained Mourrain.

"For a simpler comparison, imagine scanning an entire transparent human being and being able to visualize and monitor every single cell in his or her body,"

Zebrafish demonstrated what the authors described as "slow bursting sleep" and "propagating wave sleep," which they say are comparable to deep sleep and rapid eye movement sleep. They also found hormone signals that control propagating wave sleep, which mirrored sleep in mammals.

"These observations suggest that common neural signatures of sleep may have emerged in the vertebrate brain over 450 million years ago," the authors wrote.

"We truly did not expect to find so many similarities with human and mammalian sleep," said Mourrain.

"To see, in a live vertebrate, the complex choreography of brain and muscle activity during wake-sleep transitions and sleep was mind-blowing," he said.

However, as the authors could not keep the fish asleep more than two hours under a microscope, they couldn't monitor a full night's rest.

"Still, 2 hours is a long time, it's more than a 90 minutes NREM [Non-rapid eye movement sleep REM [Rapid eye movement] cycle in humans or it's as much sleep as a horse or elephant get during a 24-hour cycle so it's a very rich time window even if it's not the entire night," said Mourrain.

The findings could help to develop drugs and tests to treat sleep problems in humans.

"People forget that vertebrates are all very similar in their body organization and organs," said Mourrain. "We not only share a backbone protecting our spinal cord, the rest of the brain and neurochemistry allowing neurons to communicate is extremely conserved."

"Zebrafish have already been instrumental in other medical fields such as cardiovascular systems and oncology. Now they should be leveraged for sleep disorders, neurology and psychiatry at large" Mourrain argued.

Dr. Ivana Rosenzweig, a sleep physician and lecturer in the Neuroscience of Sleep at King's College London, who was not involved told Newsweek the results were "very exciting" and described it as a "significant, well-conducted study."

"The study's observations take an important step further than any of the studies so far in this animal species.

"In recent years, there has been some debate whether sleep is truly required for functioning in all animal species, and this study, albeit indirectly, might help to demonstrate a strong evolutionary drive for important sleep rhythm signatures."

However, Rosenzweig highlighted the results are preliminary and yet to be independently replicated.

"In addition, it would be a stretch to conclude that the similarity between neural signatures of slow wave sleep and dreaming sleep rhythms between zebrafish and humans also dictates their functional similarity, or that these sleep rhythms play a similar role in zebrafish and humans," she said.