Synonyms for Slumberous (same or very similar meaning)
WordNet sense 1 (quiet and tranquil):
slumbrousWordNet sense 2 (characterized by an absence or near absence of agitation or activity):
quietWordNet sense 3 (inclined to or marked by drowsiness):
somnolent, slumberyWordNet sense 4 (in a state of sleep):
asleepFrom the ODE community, based on WordNetadd/edit