Informal:
* Hit the hay: This is a classic slang term, meaning to go to bed.
* Turn in: This implies going to bed for the night.
* Get some shut-eye: This emphasizes the act of sleeping.
* Catch some Z's: This is a playful way to say "go to sleep."
* Crash: This implies going to sleep suddenly or unexpectedly.
* Go to beddy-bye: This is a child-like term.
More casual:
* Knock out: This implies falling asleep quickly.
* Get some rest: This is a general term for going to sleep or relaxing.
* Get some sleep: This is a simple and direct way to say "go to sleep."
* Go to sleepies: This is a playful term for going to sleep.
Regional:
* Hit the sack: (US)
* Go to the land of nod: (UK)
* Get some beauty sleep: (Widely used)
The best slang term to use depends on the situation and the people you are speaking to.