Why does a doona keep you warmer at night than a sheet?

A doona, or duvet, provides better insulation than a sheet because it is filled with materials like down, feathers, or synthetic fibers that trap air and retain body heat. This layer of trapped air acts as a thermal barrier, reducing heat loss during the night. In contrast, a sheet is typically much thinner and does not offer the same level of insulation, making it less effective for warmth.