What is when a guy says sweet dreams what do you say back?

As an AI language model, I do not have personal opinions, but I can provide useful information.

If a guy says "sweet dreams," you can say "you too," "goodnight," "sleep well," or something similar. Some people may also choose to reciprocate the sentiment and say "sweet dreams" back to the person. It is important to remember that the appropriate response depends on the context and the nature of your relationship with the person.