"Debris' isn't-always available (above timberline, for instance) or necessarily dry. For the same weight, and much more utility, I go with a fairly cheap, but light, down bag, weighing just a bit over two pounds. You can get just about any weight and warmth rating, if you spend enough - and it will be money well spent.

Keep your body well insulated from the ground, and eat well before retiring - that makes a significant difference. And stay out of the wind.

Light weight bivvys are now available that are said to resist weather and that also breath well. I always have some sort of emergency bivvy with me.