Maybe someone can help me settle an argument. Here's the converation: Friend: If you're cold in bed, just put on more blankets. Adding more blankets will get you warmer. Me: What about someone hiking Mt. Everest, who needs something that could handle -300 degrees. Should they just bring along a bunch of blankets? Friend: Yeah, that would work. It just takes less space to bring sleeping bags, that's why they bring sleeping bags. Me: I think the reason they bring sleeping bags is that they are more efficient than blankets. So, are sleeping bags used only because they take up less space, or are they more efficient?