Jess, I'm not sure your argument, "I sleep better in a nighty", is going to sit well with her. And, I'm having a hard time believing u can't find a substitute.
Why is THAT the only item she objects to u wearing? I'd like to hear her answer to that. Maybe it's just because that's the one thing u want? This sounds like a pissing contest with your SO.
I'm guessing u both have other unresolved issues. Maybe this nighty thing is just the one thing u both decided to hang your hats on?