I always slept in big comfortable pants, or just my undies in summer, a shirt when I'm cold, and I still do. I never sleep nude though. (Not that I am a girl, but even when I pretended I was I slept like this, so...)
But isn't 'sleep clothing' something androgynous/unisex/whatever? I mean... why would girls wear something else than guys? As long as it's comfortable, right? 😉