The Collaborative International Dictionary of English v.0.48:

Somnific \Som*nif"ic\, a. [L. somnificus; somnus sleep + facere
   to make.]
   Causing sleep; somniferous.
   [1913 Webster]

WordNet (r) 3.0 (2006):

somnific
    adj 1: sleep inducing [syn: soporific, soporiferous,
           somniferous, somnific, hypnogogic, hypnagogic]