Why is the ninth month called September?
1786, "walking in one's sleep or under hypnosis," from French somnambulisme, from Modern Latin somnambulus "sleepwalker," from Latin somnus "sleep" (see Somnus) + ambulare "to walk" (see amble (v.)).
Originally brought into use during the excitement over "animal magnetism;" it won out over noctambulation. A stack of related words came into use early 19c., e.g. somnambule "sleepwalker" (1837, from French somnambule, 1690s), earlier somnambulator (1803); as adjectives, somnambulary (1827), somnambular (1820).
somnambulism som·nam·bu·lism (sŏm-nām'byə-lĭz'əm)
a behavioral disorder of sleep in which a person sits up and performs various motor actions, such as standing, walking about, talking, eating, screaming, dressing, going to the bathroom, or even leaving the house. The episode usually ends with the sleepwalker returning to sleep, with no subsequent memory of the episode. Sleepwalking is most common in children, though it may also appear in adolescents and young adults. It occurs only during deep sleep, when dreams are basically absent. Sleepwalking becomes dangerous only when the possibility exists of the sleepwalker accidentally injuring himself.