hit the sack

VERB
12 LETTERS · STARTS WITH H

hit the sack is a valid 12-letter word worth 22 points in Scrabble and 21 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
H4I1T1T1H4E1S1A1C3K5
4 + 1 + 1 + 0 + 1 + 4 + 1 + 0 + 1 + 1 + 3 + 5 =22 PTS
DOUBLE WORD → 44TRIPLE WORD → 66
MEANING

What does hit the sack mean?