unihemispheric sleep has never been observed in owls, which might have something to do with the way their front-facing eyes are wired to their brains. In other birds, each half of the brain receives input from the opposite eye. In owls, each hemisphere receives visual input from both eyes. “So, when owls sleep with one eye open, both the left and right sides of the brain receive similar amounts of visual input,” he says. “In theory, this may cause both hemispheres to sleep less deeply than when both eyes are closed,” but this has never been proven.
