For me, as long as I'm not wearing jeans or rough trousers (such as some of the less refined woolen trousers available) I'm comfortable enough to sleep. I don't feel obligated to change from say, a loose pair of khakis to pjs or sweats. Something like your average pair of Dockers or Dickies is comfortable enough for me that I can sleep without problems.
So no, I don' change into shorts or sweats. My carryon space is often limited by business or school materiel, so I don't have a lot of extra room.
Chuck