But, as others have said, it will vary on the ride in question.
On K&SW 600 I didn't need to carry anything as the control had mattresses, blankets, etc. I think I changed my shorts but that was just because they were there (I think I carried them rather than there being a drop bag), I wasn't cold or damp so I didn't need to.
On BCM I never got there early enough to use the rest facilities, instead I'd had a power nap on a bench outside Penrhyndeudraeth.
LEL 2009 I had a inflatable mattress (in my bag drop) I got to use at Thorne twice (by sheer luck rather than planning). Other than that I slept on a tiled floor with a blanket (Eskdalemuir) and the absolute luxury of a bunk with a duvet (Alston YH).
Severn Across 400 I used to catch an hour or so on the carpeted floor of the restaurant at the services.
On more than 10 rides I've carried a silk sleeping bag inner but never actually used it. I don't struggle with sleep or cold that much. Racing snakes may have a lot more problems in this regard.