In De somno et vigilia, Aristotle states that sleep is an incapacitation of the first sense organ that occurs when the capacity for sensation has been exceeded. In the same treatise, however, Aristotle also mentions the phenomenon of motion and other waking acts performed in sleep and claims that sense perception is a necessary condition for such acts to occur. When the medieval exegesis on the Parva naturalia evolved in the thirteenth century, how Aristotle’s remark on motion in sleep could be reconciled with his definition of sleep as an incapacitation of the senses became one of the most frequently discussed problems. This article analyzes the theories on this subject in the most influential commentaries on Aristotle’s treatises on sleep and dreaming in the thirteenth century.