I sleep in the nude since I'm 8 or something like that. The only time i wear shorts to bed is when I'm not
sleeping in my own bed.
Never ever in underwear, don't you girls and guys know it's just healthier to let all your parts get some fresh(er) air now and then.
I live where the summers are hot and the winters freezing cold, but thats solved by some extra blankets in winter and silk sheets in summer. My wife also sleeps nude for as long as I know her.
Sheets get swapped a couple times a week, but it's not because we worry that we touched them during sleep, we manage to get them dirty in other ways.
Heck, do you change your pillow every day? Because I would be more worried over drool and invisible pieces of hair on the place where I put my head down than I would be worried about my ass touching some invisible skinflakes.
If you leave visible marks on your sheets just by sleeping one night in them I would consult a doctor, or a shower.