I was having a discussion with my wife about the use of the flat sheet. I say it is used as a barrier between you and the comforter, you sleep under the flat bed sheet. She says that the flat sheet is to remain tucked in and that one is not supposed to sleep under, but over, the flat bed sheet. I understand that some people don't use a flat sheet, which if fine, but when someone does use a flat sheet is it proper to sleep on top or underneath the flat sheet?