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