Well, going on Nine's northern accent, I would picture him saying 'in the niff'. But that's because I am a northerner too and it is what we say round here. But, technically the Doctor isn't a northerner - lots of planets have a north - LOL! So maybe 'in the buff' would be better. I think Rose would say 'in the buff', either that or 'kit off' as Mickey said.
I think Nine is quite plain speaking, he wouldn't try to dress it up (pardon the pun) he'd just say "I sleep naked". Anything else would sound a bit coy coming from him. But that's only my opinion.
I love the educational answers to these questions. As I have a LOVELY British beta I rely on her to set me straight when I wander astray into Americanisims so don't as a general rule think about these kinds of issues. But I adore finding out all the answers when you ask and I know at least a couple of times they have come in handy as I have used an answer for my own writing.
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Regional speech is fun!
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Wonderful
THANKS!