1
general::
Also, hit the sack. Go to bed, as in I usually hit the hay after the eleven o'clock news, or I'm tired, let's hit the sack. The first colloquial expression dates from the early 1900s, the variant from about 1940.
American Heritage Idioms
2
general::
Phrase(s): hit the hay [and] hit the sack
Fig. to go to bed. • I have to go home and hit the hay pretty soon. • Let’s hit the sack. We have to get an early start in the morning.
McGrawhill's American Idioms And Phrasal Verbs