hit the sack

Hit the sack is the idiom used when you are talking about going to bed. Hitting the sack is pertaining to your head hitting the pillow (which several decades ago meant someone using a sack for a pillow) whether this meant you are lying down or going to asleep. Nowadays, this phrase is commonly used when you are about to go to sleep. You may also hear it spoken as hit the hay which dates back to people laying in hay or straw.