Formal:
* Idę spać (pronounced "ee-day spach") - This is the most common and literal translation, meaning "I am going to sleep."
* Kładę się spać (pronounced "klah-day seh spach") - This translates to "I am laying down to sleep."
Informal:
* Idę spać (same as above) - You can use the formal translation in informal situations as well.
* Idę do łóżka (pronounced "ee-day do wush-ka") - This means "I'm going to bed."
* Idę spać/leżeć (pronounced "ee-day spach/leh-zhech") - This means "I'm going to sleep/lie down."
Other ways to express the idea:
* Dobranoc! (pronounced "doh-brah-nots") - This means "Good night!" and is a common way to say goodbye before going to bed.
* Idę odpocząć (pronounced "ee-day od-po-chach") - This means "I'm going to rest."
You can also use these phrases in the third person, for example:
* Ona idzie spać. (She's going to sleep)
* On kładzie się spać. (He's going to bed)
Remember that the best way to choose the right phrase depends on the situation and your relationship with the person you're talking to.