the sleeping bag could be part of a shelter system, rather than viewed as a stand alone. Perhaps you start with a lighter bag-say 20 degree capability, given that the ratings are assigned by marketers and have no objective meaning. The sleeping bag prevents conductive and convective, and maybe radiational heat loss on top of you, and its effectiveness is direc
tly related to thickness and trappe-air content. Under you, your butt compresses the insulation and markedly decreases its effectiveness. The ground is typically cold and wet, so you want a relatively non-compressible layer of insulation-foam pad, dry forest stuff, and a waterproof, reflective layer-space blanket, poncho, tarp.If it's colder, you can add a sleeping bag liner (down, fiberfill, fleece), a bivi sack cove (gore-tex, microfiber) or a poncho-poncho-liner combination. a tarp, or tent, or improvised shelter reduces convective loss, and keeps the rain-snow-sleet off your face. there are some interesting modular bag systems available that contain outer ba, inner bag, bivi sack combinations. The big agnes line of bags save some weight by putting no insulation on the bottom, assuming that youy will add a foam pad.
or you can do the canvas bedroll and wool blanket deal that the cowboys used. Unlike miracle fabrics, it won't diappear if you get it too hot, and will wear long enough to be part of your estate. heavy, tho