noun
sleepwalker; somnambulist
Medical or formal term for a person who walks while asleep. In everyday conversation, 夢遊病の人 or 睡眠時遊行症の人 may be used instead.
彼は夢遊病者で、夜中に家の中を歩き回ることがある。
He is a sleepwalker and sometimes walks around the house at night.
Compound of 夢遊 (sleepwalking) + 病 (illness) + 者 (person). The exact historical derivation is uncertain, but the word follows a common pattern for naming a person with a condition.