Both are absolutely correct, and as usual the one with subject/verb inversion is more formal, the other one more casual, but for this precise sentence, the difference I would say is quite minimal.

You would say: «Avez-vous bien dormis?»

This is another formal way. - Est-ce que vous avez bien dormis ?

"Avez-vous bien dormi?"

Hello Kelly, To be exact, the difference is in the punctuation. I explain : • "Avez-vous bien dormi ?" Here, even without the interrogation point the sentence already is a question. But it is not a reason to forget it. However, when I say : "Vous avez bien dormi", whithout an interrogation point the sentence is simply a finding. To make about it a question it have to end the sentence with an interrogation point : "Vous avez bien dormi ?" And to be sure do not make mistake, you can take the example from Rocio : Est-ce que vous avez bien dormi ? Have a nice day, Christian