Of the type I like big agnes has them , the mattress an insulated air mat,
it is the bottom of the bag , slides in a pocket, or a zip connected, to the top.

You can turn over inside the bag, rather than turning over the whole mummy cocoon .
bottom of bag insulation is compressed, by your body weight, as it is.

so insulated air mat, as the bottom, takes care of 2 things at once..