Source : Webster's Revised Unabridged Dictionary (1913)

Somniferous \Som*nif"er*ous\, a. [L. somnifer; somnus sleep +
   ferre to bring.]
   Causing or inducing sleep; soporific; dormitive; as, a
   somniferous potion. --Walton.

Source : WordNet®

somniferous
     adj : sleep inducing [syn: {soporific}, {soporiferous}, {somnific},
            {hypnogogic}, {hypnagogic}]