Depending on the temperature and my mood:
Boxer shorts and t-shirt
Just boxer shorts
Naked
I wear briefs by day but I do not like to sleep in briefs, I like to "air out", so to speak.
Kind of a funny story, I have an aunt who used to always buy me pajamas for Christmas, long after I stopped wearing pajamas (but she did not know that). Some needy men got some really fine, brand new pajamas shortly after Christmas!