The top sheet is a casualty of the duvet. Traditional British and American beds were built in layers: a fitted sheet, a flat top sheet, blankets, and sometimes an eiderdown over the lot. The continental duvet replaced all of that with a single quilt inside a washable cover, and it reached Britain commercially through Terence Conran, who had come across duvets in Sweden in the 1950s and began selling them at his Habitat store in London from 1964, promoted as an alternative to the sheet-and-blankets bed. Once the cover itself goes through the wash, the hygienic job the top sheet used to do (keeping the blankets off you) disappears, which is why the people who skip it think it is redundant and the people who keep it think it is basic hygiene. The divide runs by age rather than by country: YouGov finds about one in eight American adults use no top sheet at all, and those who do use one split fairly evenly between tucking it in and leaving it loose.