A somnambulist is a person who walks about while they are asleep — a sleepwalker. The -ist ending marks the PERSON; the condition itself is somnambulism. Like its partner word, this one is formal and rather literary. In real speech people say sleepwalker; somnambulist turns up in medical writing, in old novels and in films, where it often carries a slightly eerie feeling, as though the person were under a spell rather than simply asleep. That flavour comes partly from the famous 1920 German film The Cabinet of Dr Caligari, where a somnambulist is controlled by another man. Writers still borrow the word for that reason. You may also see it stretched: someone moving through life without noticing anything, doing everything on habit, can be called a somnambulist — 'he went through his job like a somnambulist'. That use is figurative and always a little bit critical.

夢遊者、夢遊症患者（正式或文學用語；日常講 sleepwalker）