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Neural activity in a zebrafish transitioning from waking to sleeping. Credit: Leung et. al. Nature 2019.

Most animals sleep, but scientists still lack a complete understanding of why, the biological factors that regulate sleep cycles and how the behavior evolved.

Driving the news: A new study in Nature on tiny zebrafish finds how humans sleep today may have first evolved in vertebrates more than 430 million years ago.

Why it matters: Scientists hope a better understanding of how zebrafish sleep, down to the cellular level, could unlock avenues for new treatments of sleep disorders that affect 50-70 million Americans but are often undiagnosed. It could also improve their understanding of sleep's role in memory and our physical health.

What they did: Zebrafish lack a neocortex — the part of the mammalian brain involved in higher order functions such as sleep and the target of sleep studies — so the scientists from Stanford University and institutions in Japan and France needed to find a workaround.

  • They developed an imaging platform that allowed them to view proxies for brain and muscle activity via fluorescence, as well as heart rate and eye movement.
  • They used two-week-old zebrafish since they are transparent, and therefore ideal for fluorescent imaging techniques.
  • They also performed other experiments to determine the effects of particular compounds on zebrafish sleep and wake cycles.

What they found: Ependymal cells — present in humans and zebrafish, and known to play a role in the brain and spinal cord — are among the first to be activated as the fish fall asleep.

  • The researchers also found human hypnotics — compounds found in sleeping pills and anesthetics — can also induce sleep phases in the fish that are analogous to humans.
  • And similar to what is seen in humans, chemicals released by neurons in the zebrafish brain regulate the creature's muscles and sleep.

What's new: Scientists had observed invertebrates (octopi, insects) and vertebrates (fishes, amphibians, birds, mammals) sleeping but the physiological signatures of sleep, like rapid eye movements, had only been observed in mammals, birds and reptiles, study co-author Philippe Mourrain of Stanford University tells Axios.

What they're saying: “You can’t just say sleep is sleep,” Jerry Siegel, a sleep scientist at UCLA who was not involved in the study, told National Geographic. He cautioned that connections between sleep in young zebrafish and sleep in humans are less straightforward than the study suggests. For example, he said, many mammals lack REM sleep completely, and some mammals sleep 20 hours a day whereas others just need 6 to 8 hours or less.

The study's lead author, Louis C. Leung of Stanford, tells Axios:

"I encourage everyone to cherish what has taken 100's of millions of years to create and hope there is soon a change in the public narrative regarding the importance of sleep — we should be proud not embarrassed to get enough sleep."

What's next: Developing animal models that examine sleep functions at the cellular level, like the one in this study, could lead to treatments for sleep disturbances that are linked to psychiatric disorders as well as Parkinson's and Alzheimer's.

Go deeper: Americans are sleeping more

Go deeper

Updated 5 mins ago - Sports

IOC: Belarus sprinter who sought refuge in Tokyo "safe"

Krystsina Tsimanouskaya of Belarus in 2019. Photo: Ivan Romano/Getty Images

Belarus' Olympian Krystsina Tsimanouskaya, who sought refuge in Tokyo, is in the care of Japanese authorities and the UN refugee agency is now involved in her case, an International Olympic Committee official told reporters Monday.

Driving the news: The sprinter said she wouldn't obey orders and board a flight home after being taken to Tokyo's s Haneda airport by team officials Sunday following her criticism of Belarusian coaches, per Reuters. She spent the night in an airport hotel.

Updated 38 mins ago - Sports

Olympics dashboard

Italy's Lamont Marcell Jacobs of Team Italy crosses the finish line ahead of American Fred Kerley in the men's 100m final on day nine of the Olympic Games at Olympic Stadium in Tokyo, Japan, on Sunday. Photo: Cameron Spencer/Getty Images

🚨: IOC "looking into" American Raven Saunders' Olympic podium gesture

🏃🏾: Italy's Lamont Marcell Jacobs: Reconnecting with U.S. father "gave me the desire to win" Olympic 100m sprint race.

🥇High jumpers persuade Olympic officials to let them share gold

🏌️‍♂️: Golfer Xander Schauffele wins gold for U.S. by one shot

🤸🏿‍♀️: Simone Biles won't compete in Olympic floor finals, individual vault or uneven bars

🏳️‍⚧️: Axios at the Olympics: Games grapple with trans athletesTrans athletes see the Tokyo Games as a watershed moment

Go deeper: Full Axios coverage

Updated 51 mins ago - Sports

IOC "looking into" American Raven Saunders' Olympic podium gesture

Team USA's Raven Saunders gestures on the podium with her silver medal after competing in the women's shot put event during the Tokyo 2020 Olympic Games at the Olympic Stadium in Tokyo on Sunday. Photo: Ina Fassbender/AFP via Getty Images

The International Olympic Committee is "looking into" U.S. shot-putter Raven Saunders' gesture on the Tokyo Games podium after she won a silver medal, IOC spokesperson Mark Adams told reporters Monday.

Why it matters: Saunders told AP she placed her hands above her head in an "X" formation while on the podium to stand up for "oppressed" people. The IOC has banned protests during the Tokyo Games.

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