loose boxers (tight ones get a bit sweaty), and an old t-shirt if it gets cold.
Biggest problem I have is getting a properly comfy t-shirt. Can't be too loose, too clingy, too thick and has to have the right shape collar.
Used to sleep nekkid, but having kids (and an aging bladder) means I'm often up a few times in the night and as we rarely shut the curtains and the house is overlooked I don't want to give the neighbours a free show
Edit - not bothered about the kids seeing my nekkid, they're used to that.