Formal:
* Сладких снов (Sladkikh snov): This is the most common and literal translation, meaning "sweet dreams."
* Приятных снов (Priyatnykh snov): This means "pleasant dreams" and is a slightly more formal option.
Informal:
* Спокойной ночи (Spokoynoy nochi): This literally means "good night" but is often used as a way to say "sweet dreams."
* Добрых снов (Dobrykh snov): This means "good dreams" and is a friendly way to say it.
Other options:
* Сладких снов и доброго утра (Sladkikh snov i dobrogo utra): This means "sweet dreams and good morning."
* Пусть тебе приснится что-нибудь хорошее (Pust' tebe prisnitsya chto-nibud' khoroshee): This means "May you dream something good" and is a more personal way to say it.
The best option will depend on the context and your relationship with the person you are talking to.