In the nude all year round. If the person sharing the bed or the room with me is uncomfortable with me being nude (has happened more than once) I'll put on a t shirt and sleeping pants, but I simply can't stand to sleep in boxers, briefs or anything tight around the crotch.
Sleeping has recently become a priority, when I realised my inconsistent sleeping habits were basically destabilizing my entire routine. That means a minimum 8 hours/night for me now, and 9 or more whenever I get the chance. Definetely recommend anyone who's in need of some positive change to look into their sleeping hours..