Anyone who prefers to sleep under a down or down alternative duvet insert should plan to buy a duvet cover. Unlike comforters or blankets, duvet inserts generally aren't designed to be washed frequently, as doing so can damage down over time, cause the fill to get lumpy, or even melt synthetic fibers. It's especially important for anyone who prefers to sleep without a top sheet to get a duvet cover to protect their duvet insert. Consider it an investment that, over time, will pay off by extending the life of your duvet insert.