somnambulismus

Somnambulism combines Latin somnus ('sleep') and ambulare ('to walk'). The compound was constructed in the 1680s, appearing in medical Latin texts as somnambulismus. The word described exactly what it said: walking in one's sleep. But its meaning would take a long detour through mesmerism before returning to its origins.

In the 1780s, the Marquis de Puységur, a student of Franz Anton Mesmer, discovered that some mesmerized subjects entered a trance state he called 'magnetic somnambulism.' These subjects walked, talked, and responded to instructions while appearing to be asleep. Puységur's term hijacked the word: for the next century, somnambulism in medical literature more often meant 'hypnotic trance' than 'sleepwalking.'

James Braid reclaimed the territory in the 1840s by coining the term hypnotism (from Greek Hypnos) as a replacement for 'magnetic somnambulism.' Braid's new term gradually took over, freeing somnambulism to return to its literal meaning. By the early 1900s, somnambulism again meant sleepwalking, and hypnotism described the trance state.

Modern sleep science classifies somnambulism as a disorder of arousal, occurring during slow-wave (N3) sleep. It affects up to 15 percent of children and 4 percent of adults. In rare cases, sleepwalkers have driven cars, cooked meals, and committed violence with no memory of doing so. The legal defense of somnambulism—automatism—has produced acquittals in murder cases, including the 1987 case of Kenneth Parks in Canada.