WordNet sense 1 (a drug that induces sleep):

hypnotic

WordNet sense 2 (sleep inducing):

somniferous, hypnagogic, hypnogogic, somnific, soporiferous

WordNet sense 3 (capable of depressing physiological or psychological activity or response by a chemical agent):

depressant

WordNet sense 4 (inducing mental lethargy):

narcotic