Fish Seem to Nap Like Humans

Fish may lack eyelids, but when it comes to sleep, a new study suggests they're a lot more like humans than you might guess. The study in Nature Communications finds zebrafish cycle through four distinct sleep "substates" that parallel human sleep stages—right down to something resembling naps, reports the New York Times. For their study, researchers at Germany's Max Planck Institute used a custom microscope and camera system to track eye movements and brain activity (the fish are nearly transparent in their early larval stage) of 105 zebrafish in a tank.

The researchers saw 10 hours of nighttime sleep divided into three stages, from a deep, glassy-eyed phase to lighter phases where eyes twitch or drift to one side. A fourth state appears in daytime bursts lasting up to 10 minutes: the fish's eyes scan as if on alert, but brain activity plunges, and waking them is tough. The work suggests key sleep mechanisms are consistent across species, with many of the same genes, neurons, and drugs influencing slumber in both fish and humans.