Hi,
That's right, you can keep having sex and other things with Yuna after she wakes up during nightly fun, only if you have max relationship (100 or 10/10 hearts).
Moreover, if you haven't seen already, there's also a scene that triggers after about 5mins after you wake her up during the night. Afterwards the night just ends.
Either way, when she wakes up it's still technically "night over" as you don't get any credits for that night.
Cheers