It took a decade’s worth of work—and probably a few sleepless nights—but for the first time, researchers have identified sleep patterns in the brains of tiny zebrafish, and those patterns look remarkably similar to the brain activity in sleeping humans.

As scientists report today in the journal Nature, evidence of similar sleep patterns in both fish and mammals may offer clues about the evolution of sleep in our common ancestors, which could in turn help us better understand the biological function of nodding off.

“Sleep is a huge mystery in neuroscience,” says William Joiner, a biologist at the University of California, San Diego, who studies sleep in fruit flies but was not involved with this research. Plenty of