The pupil not only regulates the amount of light entering the eye, but also reflects the state of the brain: when we are aroused or focused, the pupils dilate. The pupil even changes as mammals sleep, constricting during deep non-REM sleep and dilating in REM sleep. Now, for the first time, researchers at the Max Planck Institute for Ornithology and the Neurosciences Research Center in Lyon have studied pupil behavior in birds. Surprisingly, the exact opposite happens here: During arousal and REM sleep, the pupils become smaller, while they enlarge during non-REM sleep. The unexpected pupillary behavior of birds provides a new window into the sleeping brain.