WordNet sense 1 (a downward slope or bend):

descent, fall

WordNet sense 2 (assume a reclining position):

lie down

WordNet sense 3 (take a seat):

sit down

WordNet sense 4 (move downward and lower, but not necessarily all the way):

descend, go down

WordNet sense 5 (disappear beyond the horizon):

set

WordNet sense 6 (become smaller):

wane

WordNet sense 7 (prepare for sleep):

bed, retire, go to bed, turn in, crawl in, go to sleep, hit the hay, hit the sack, kip down, sack out