In a curious and much-discussed passage in De insomniis, Aristotle claims that if a woman looks into a mirror during her menstrual period, her gaze will cause a blood-red stain to appear on the mirror’s surface. Modern commentators have struggled with the passage and its authenticity has been both questioned and defended. In the medieval Latin reception of Aristotle’s treatises on sleep and dreams, many of the commentators devote considerable attention to Aristotle’s account of the mirror phenomenon and try to harmonise it with his overall theory of perception as well as with his theory of motion. The paper analyses the development in a number of Latin commentaries on De insomniis from the twelfth and thirteenth centuries.