Something like an
electric blanket, only it fits neatly around your
mattress and heats from beneath you, which only makes sense considering that
heat rises. It is one of three things that tend to make it very difficult to
get out of bed on
frosty mornings here in
Michigan, the other two being
flannel pajama bottoms and a
snuggly bed partner.
Electric mattress covers are washable, available to fit various bed sizes, and sometimes come with dual controls. You can find them at JC Penney and probably other department stores.