Hit the hay

To 'hit the hay' means to go to bed.  It recalls a simpler time when people may have slept in a barn.

Example:

-Okay, kids!  Time to hit the hay!

-Aw!  5 more minutes?

Slept like a rock

If you 'slept like a rock,' it means that you had a great night's sleep and feel well-rested.

Example:
-How did you sleep?
-I slept like a rock! I feel great!