WordNet sense 1 (sleep inducing):

soporific, somniferous, hypnagogic, hypnogogic, somnific

WordNet sense 2 (capable of depressing physiological or psychological activity or response by a chemical agent):

depressant

WordNet sense 3 (inducing mental lethargy):

narcotic

WordNet sense 4 (arousing no interest or attention or curiosity or excitement):

uninteresting