Formal:
* Breuddwydion melys i chi: This is the most literal translation and means "sweet dreams to you." It's a very polite and formal way to say it.
Informal:
* Breuddwydion da i chi: This is a more casual way to say "good dreams to you." It's a common way to say it between friends and family.
* Cysgu'n dda: This means "sleep well" and is another common way to say goodnight in Welsh.
Romantic:
* Breuddwydion melys: This means "sweet dreams" and is a more romantic way to say it. It's often used by couples.
Other options:
* Hoffech chi freuddwydion da? This literally means "Do you wish for good dreams?" It's a more informal way to say it.
* Gobeithio y byddwch chi'n cael breuddwydion da: This means "I hope you have good dreams." It's a very polite and thoughtful way to say it.
Ultimately, the best way to say "sweet dreams" in Welsh depends on the context and the relationship you have with the person you're talking to.