Hit the sack means when you feel exhausted, and too tired just go to bed or go to sleep.
Situation : Peter returned home late night. When wife called him for dinner he says " I am too tired and want to hit the sack right now".
Situation : Mother saw Anand was awake till 2:00 a.m studying for next day exam.
She says " Anand, it is pretty late now; you better hit the sack now and finish the rest in the morning.