how did it develope?
in the mid ages, because of the backward productivity, the bed is very expensive, people can't afford their own bed, they can only share same bed with others. with the developement of techonology and productivity, the price of a bed droped, have a personal bed become possiable. in this stage, the concept of personal space become stronger. people can use batter material to make the mattresses and use many layers of mattresses to make sleep more comfortable. rich people will use many expensive material to make the curtain of the bed. today, with the improvement of techonology, our mattresses is much more stable than old mattresses, the modern bed frame are also stronger than before.