Most body heat lost in a sleeping bag is due to heat conduction, or heat transfer caused by physical contact to the ground below. The weight of the sleeper compresses the sleeping bag underneath thereby greatly reducing the bag’s thermal efficiency. A well-insulating ground pad is very important to help reduce this heat loss.
This is the heat loss due to air currents that carry body heat from areas of high temperature to those that are lower. The more room around the body, or space within the bag, the greater this heat exchange.
In colder temperatures, the body loses heat and moisture through breathing.
Radiation is the heat lost to objects in the surrounding environment. Wind reinforces this effect. It is therefore best to choose a sleeping area that is protected from wind.
The human body sweats in order to regulate body heat. This evaporative cooling process is an effective reaction when your body attempts to cool itself down, yet ineffective if the sweat is not able to evaporate away from your body. If a sleeping bag is not breathable enough, the bag soon becomes wet and uncomfortable. This causes you to feel cold and makes the sleeping bag less effectual in keeping you warm. This is a greater problem in down sleeping bags, which absorb moisture to a higher degree, than in synthetic sleeping bags.