The phrase "After a long day, I couldn't wait to hit the sack" is an idiom. It uses figurative language to convey the idea of going to bed to sleep without literally referring to hitting a sack.
- Personification involves giving human traits to non-human things.
- Hyperbole is an exaggerated statement not meant to be taken literally.
- Onomatopoeia refers to words that imitate sounds.
- Alliteration involves the repetition of the same initial consonant sound in a series of words.
In this case, the phrase fits the idiom category best.