Hitting the Sack

As detailed in "hitting the hay," sacks filled with hay were once used as mattresses in the United States. Thus, the expression "hitting the hay" came into wide use as a metaphor for going to bed and going to sleep. Ex.: "I'll hit the sack after I finish watching Jay Leno." (Jay Leno's comedy show is a late night television program, so anyone watching it is automatically up late to begin with.)