Undies only - I don’t own any “night attire”.
I just have to hope that my house doesn’t catch on fire while I’m asleep… could provide an eyeful for the neighbours… 
re: sheets being the same as underwear…um… I don’t generally have the sheets pressed up against my vagina when I sleep, or wedged between my ass cheeks. Not sure how you sleep.
Naked, always, since I was 10. Even on overnight school trips (though I’d often leave my boxers on until I was under he covers). I picked it up at summer camp, Christian summer camp . Even if it’s really cold I’m far more comfortable putting a heavier blanket on the bed and/or turning the heat up than putting anything resembling clothing on.
Naked unless it’s That Time Of the Month then it’s naked with panties unless it’s also winter then it’s naked with socks. And panties. Unless it’s Not That Time Of The Month, then it’s just naked with socks. But only in winter. Otherwise, it’s just naked.
I sleep nude, except on the coldest nights. If I wear something, it drags on the sheets when I roll around, and next thing I know, I’m tangled in the sheets and blankets. Strangely, though, I have to have something covering me or I can’t sleep (I think it’s because I don’t like feeling breezes when I’m trying to sleep); if I’m nude I have to have at least a single sheet over me, even when it’s really hot, and of course I keep all my clothes on if I flop on the bed for an afternoon nap.
Jammies. Always. I have lots of pjs – from flannels with scottie dogs dancing on them, to slinky slippery black wisps. The semi-SO thinks the black wisps are a LOT more interesting than simple naked flesh, bless the boy. I haven’t shown him the flannels yet, but he might find those fetching as well . . .
I sleep in pajamas, always.
I would feel too… naked without clothes on.
Naked almost always
Almost always naked, sometimes with boxers. If it’s really cold, I’ll wear a t-shirt to spare my shoulders from the chill. Sometimes I’ll leave my socks on; I have long legs, and if I tuck the sheets under the bed my feet and legs feel really restricted and I end up kicking them out anyway. The socks keep me from freezing.
If I’m over at someone else’s place, t-shirt, boxers, and flannel pants.
I wear loose boxer shorts. I am confident that the proportion of my body touching my boxer shorts is equivalent of what my sheets would get if I slept in the nude – i.e. my shorts do not get wedged in between my ass cheeks. I don’t have a vagina.
So you do spooge in your underwear every day?
My sheets don’t go between my legs at all, though, which boxers do. I still think that sheets have a lot less contact with, say, potentially sweaty parts of me than any type of underwear would. For example, if I wore the same underwear (of any kind) for two weeks and then sniffed 'em, versus sniffing sheets I’d been sleeping in nude for two weeks, I am thinking there would be a significant difference.
I sleep naked; I don’t really sweat at all even when it is really humid. I also have to be as spread out as possible as I don’t like any part of my body touching any other part. If there is a fire the neighbours will just have to avert their eyes.
I will admit, my underwear comparison is closer to boxer wearing men than thong wearing women (or, uh, men, I guess).
I wear Jockey shorts to sleep in. As stated several times above, this just keeps the sheets cleaner. Wear the Jockeys during the day, sleep in them at night, clean ones in the morning. I find I can go a couple weeks without washing the sheets, as compared to sleeping in the buff, when it was washing sheets every week.
So when it was “washing the sheets every week” what was your cue that it was time to wash them? Or was it just a psychological sense of “should”?
And please, for the love of God, don’t say “skid marks.”
How about “I stick to my sheets.”
For what it’s worth I change my sheets once a week during cold weather and 2/3 times during hot weather. After sex they get changed the next morning unless the sex was in the morning or daytime then they get changed sooner.
BAND NAME! 
I’ve never had crusty sheets. And I change my sheets because it “seems like it’s probably time” not because there is any visible or tactile difference, or because of any smell. (The exception being pillow cases if I’ve gone to sleep wearing makeup, which is rare, in which case sometimes they get tiny black smudges on them.) But I do acknowledge that I’ve got a pretty strange body. I don’t tend to have body odors, even on my feet, and I barely sweat. A stick of deodorant usually lasts me several years.