WordNet sense 1 (assume a reclining position):

lie down

WordNet sense 2 (take a seat):

sit down

WordNet sense 3 (move something or somebody to a lower position):

lower, lower, get down, bring down, take down

WordNet sense 4 (put on clothes that make a careless or ill-dressed impression):

dress down

WordNet sense 5 (prepare for sleep):

go to bed, turn in