Somnambulism is walking around while you are asleep. Note carefully: this word is the CONDITION or the ACTIVITY, not the person — the person is a somnambulist. It is a formal, medical-sounding word. A doctor writing notes may use it; a sleep clinic may use it; your friend will just say sleepwalking. Both are correct English, but sleepwalking is what people actually say, and somnambulism is what appears in a textbook, a news article trying to sound serious, or an old horror story. The real thing it describes: the person gets up, eyes often open, may walk, open doors, get dressed, even eat, and afterwards remembers nothing. It is most common in children and usually stops as they grow up. Films show sleepwalkers with arms straight out in front — that is a joke from old cartoons, not real. One thing to know: waking a sleepwalker is not dangerous, though they may be confused and frightened; the safer thing is to guide them gently back to bed.

夢遊（症）（醫學正式用語；日常講 sleepwalking）