When you say that you are going to `hit the sack’, you are saying in a rather informal way that you are going to bed. What does a sack have to do with a bed? During World War II, American soldiers started referring to their sleeping bags as `sacks’. `Hit the sack’ replaced the earlier expression `hit the hay’. Why hay? In the old days, people who couldn’t afford a bed used to sleep on a sack stuffed with hay. Before they lay down to sleep, they used to keep hitting the sack till the straw/hay was evenly spread. Hence the expression `hit the hay’.
*I’ve had a tiring day. I think I am going to hit the sack early today. *You don’t seriously expect me to hit the hay so early, do you?