Formal:
* 睡觉 (shuì jiào): This is the most common and general way to say "sleep" or "go to bed".
* 上床睡觉 (shàng chuáng shuì jiào): This literally means "go to bed and sleep", and is slightly more formal.
Informal:
* 睡觉觉 (shuì jiào jiào): This is a more informal and playful way to say "go to bed".
* 休息 (xiū xi): This means "rest" and can be used to mean "going to bed" in an informal context.
* 睡 (shuì): This is a very informal way to say "sleep" or "go to bed".
Examples:
* 我要睡觉了。 (Wǒ yào shuì jiào le.) - I'm going to bed now.
* 现在已经很晚了，该上床睡觉了。 (Xiànzài yǐjīng hěn wǎn le, gāi shàng chuáng shuì jiào le.) - It's very late now, it's time to go to bed.
* 我好困，我要睡觉觉了。 (Wǒ hǎo kùn, wǒ yào shuì jiào jiào le.) - I'm so tired, I'm going to bed.
* 你要休息了吗？ (Nǐ yào xiū xi le ma?) - Are you going to bed?
The best way to choose the right phrase depends on the situation and who you are talking to.