I have been a cold sleeper my entire life. Not any worse with age. This winter, at home, after nudging from my daughter, I have started using a hot water bottle in bed. It mainly warms up the bed. Otherwise the cold sheets just suck the heat from me and it takes me hours to get warm. A similar thing happens in a sleeping bag. FIRST, you have to warm the sleeping bag with your body heat, THEN it insulates against cold. If it sucks the heat from you, then it is really hard to recover the body heat. I have hesitated using a hot water bottle because of the horrible consequence of what happens if it leaks. I am now inclined to think it just may work, at least if I keep the bottle upright and use it to warm the bag, then take it out. If you carry a dry-bag you could also put the hot water bottle inside for a back-up. And, by morning, the water would be body heat temperature or a bit less, but much warmer than a bottle sitting out, so you would use less fuel for breakfast. Just a thought.