Source : Webster's Revised Unabridged Dictionary (1913)

Soporiferous \Sop`o*rif"er*ous\, a. [L. soparifer; sopor a heavy
   sleep + ferere to bring.]
   Causing sleep; somniferous; soporific. ``Soporiferous
   medicine.'' --Swift. --- {Sop`o*rif"er*ous*ly}, adv. --
   {Sop`o*rif"er*ous*ness}, n.

Source : WordNet®

soporiferous
     adj 1: sleep inducing [syn: {soporific}, {somniferous}, {somnific},
             {hypnogogic}, {hypnagogic}]
     2: inducing mental lethargy; "a narcotic speech" [syn: {narcotic},
         {soporific}]