somniferous \som-NIF-uhr-uhs\ (adjective) - Causing or inducing sleep.
"The wine which had exerted its somniferous influence over Mr. Snodgrass, and Mr. Winkle, had stolen upon the senses of Mr. Pickwick." -- Charles Dickens, 'The Pickwick Papers'
Somniferous comes from Latin somnifer, "sleep-bringing," from somnus, "sleep" + ferre, "to bring."