Most sheet sets typically include a flat sheet, fitted sheet, and one or two pillowcases. There’s not much need for wondering if you need a flat or fitted sheet for this reason — both are useful and usually come as a package deal. Some other sheet sets, like those Casper offers, also include a duvet cover. 

However, flannel sheets can be made of cotton, wool, or synthetic fibers, leaving brushed cotton a  good balance of breathable and warm.

Bed Sheet: A bed sheet is the foundation of your bedding ensemble. This is the fabric layer that lies directly on your mattress, coming in close contact with your skin. You'll find two primary types of bed sheets: the fitted sheet, which snugly covers the mattress with its elastic edges, and the flat sheet, which rests atop the fitted sheet.

Just like with most things, though, not all cotton plants are created equal. Egyptian cotton that grows along the Nile River produces the highest quality textiles because of its extra-long staples.