The phrase has roots in both historical bedding practices and military slang:
: In the late 19th and early 20th centuries, mattresses were often just large cloth sacks stuffed with hay or straw. "Hitting the sack" was a literal description of laying down on these improvised beds.
: Some sources trace the slang use of "sack" for a bunk or bed back to naval contexts as early as 1825. Usage and Examples