Top Sheet is also known as a flat sheet under which the bed occupants lie. The https://www.aanyalinen.com/blogs/aanya-blog/what-is-a-top-sheet (flat Sheet) is generally used to protect mattresses and bed covers like duvets and comforters. In the warmer nights, the duvets can be too much for you. In those cases, the top sheet works as the cover that you need. Like an air conditioner, the top sheet catches the cooler air in and gives you a cooling down feeling.