From 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]
From WordNet (r) 3.0 (2006) :
somniferous
adj 1: sleep inducing [syn: soporific, soporiferous,
somniferous, somnific, hypnogogic, hypnagogic]