Sleep.
Latin somnus, sleep.
A somnambulist is a sleepwalker (Latin ambulare, to walk), for which the associated noun is somnambulism; in medicine, a somnifacient is a drug that induces sleep or drowsiness; something somniferous tends to induce sleep; somnolent can refer to a person who is sleepy or drowsy or to something inducing drowsiness.
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