The Collaborative International Dictionary of English v.0.48:

Somniferous \Som*nif"er*ous\, a. [L. somnifer; somnus sleep +
   ferre to bring.]
   Causing or inducing sleep; soporific; dormitive; as, a
   somniferous potion. --Walton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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