The whole point of a sleeping bag is to keep you warm and comfortable throughout the night when you sleep outdoors. The colder it is outside your
camping tent the more important it is to have a quality bag that will do its job so you can get some sleep. There is the misunderstanding that a sleeping bag is supposed to provide you heat but this is not how they actually work.
Sleeping bags, especially mummy sleeping bags are designed to retain your own body heat to keep you warm throughout the night. Therefore the better a sleeping bag fits the user the better it will retain that heat and not let any escape. That is why for cold weather campers the mummy sleeping bag is the style of choice for the warmest night possible.
Even if you crawl into a mummy sleeping bag cold it will retain the body heat you do have to begin warming you up. As your body warms so will the bag providing you with a comfortable night sleep. Most of these also include a warm hood that includes a drawstring so you can ensure that the heat of you body does not escape out the opening for your head.
