A comforter is a type of bedding made of two lengths of fabric or covering sewn together and filled with insulative materials for warmth, traditionally down or feathers, wool or cotton batting, silk, or polyester and other down alternative fibers. It is similar to a thick, fluffy blanket and is usually laid over a top bed sheet and used to cover the body during sleep. Comforters are typically used in the winter season when it is very cold, although a variety of different thicknesses means that they can be used in other seasons as well, with lighter examples being used in warmer weather. They are often sold in bedding sets and come in a variety of styles