Having a bottom sheet that sits above your mattress – and a mattress protector – is always necessary. It'll help keep your mattress free from stains and spills and give you a comfy sleep surface your body will thank you for. But most sheet sets come with a top sheet anyway, so you might as well add that to your comforter. A top sheet will act as a barrier between you and your comforter, stopping skin cells and bacteria build-up. While this means you won't have to wash your comforter as regularly as your sheets, the sheet itself isn't necessary.