I just had this discussion with my son and DIL yesterday. they don't use a top sheet because they say it pulls out of the bottom and gets all bunched up. I took the flat sheet and put elastic sleeves on it so it is fitted on the bottom. No excuse, right? Well they are now sleeping on the top sheet.
They use a heavy quilt to cover themselves. I told them that they can wash the sheets as often as they want but they wash the quilt about 3 times a year and it will get covered in body dirt, dead skin cells etc. It doesn't matter to them. YUCK!!!
I always use a top sheet, sometimes in the summer just the top sheet, no blanket. In the winter I have a fitted heated mattress pad, the sheets, a blanket and a quilt. And I am still cold.
My other son doesn't use anything, he is always hot and sleeps in boxers on top of the mattress, no blankets. Peggie