More about somnambulism
Somnambulism, “sleepwalking,” comes via French somnambulisme from New Latin somnambulismus, a pretty transparent compound of the noun somnus “sleep” and the verb ambulāre “to walk, take a walk, stroll,” source of English amble. Somnus is the Latin result of the very common Proto-Indo-European root swep-, swop-, sup– “to sleep.” In Latin, the derivative noun swepnos (or swopnos) becomes sopnos, then somnus. The derivative noun supnos becomes hýpnos in Greek. Another derivative noun, swep–os-, becomes sopor– “sleep” in Latin (via swop–os-, then sopor-), as in English soporific “causing sleep.” Swepnos becomes swefn “sleep, dream” in Old English and sweven “dream, dream-vision” in Middle English. William Langland, usually considered to be the author of Piers Plowman, fell into a merveilouse swevene, a “curious dream,” one May morning in the Malvern Hills in Hereford and Worcestershire, England, and Piers Plowman is the narrative of his dream. Somnambulism entered English at the end of the 18th century.