Source : Webster's Revised Unabridged Dictionary (1913)

Somnific \Som*nif"ic\, a. [L. somnificus; somnus sleep + facere
   to make.]
   Causing sleep; somniferous.

Source : WordNet®

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