I concur with the "Nun".
After purchasing one of those overpriced Exped downmat sleeping pads last fall, it finally dawned on me that I was already carrying all this warm clothes and that I could rely on a lighter, smaller sleeping bag if I wanted.
I made it - quite comfortably - through several teen and twenty degree nights with a 40 degree sleeping bag, while wearing some extra layers of clothes and nuzzled into the downmat. My sleeping bag now weighs one pound. I carry the same clothes as before. Seems like progress to me but I do reserve the right to change my mind, probably when the downmat deflates.