Extreme

At this temperature, the sleeping bag offers protection from freezing, however there is risk of hypothermia. It is assumed that a “standard female” in a tightly rolled-up position can withstand the cold for 6 hours.

THERMAL REGULATION

When you’re sleeping, your thermal regulation depends on various physical factors which should be taken into account when choosing your sleeping bag.

Tip: The functionality of your sleeping bag is closely related to the functionality of the clothing you’re sleeping in. Functional apparel such as a down jacket can significantly increase the thermal performance of a light sleeping bag while keeping the overall weight of your gear to a minimum.

Heat Conduction

Most of the 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.

Convection

Air is always in motion. Warm air rises due to its lower density, while cold air is heavier and sinks to the ground. This results in changes in air pressure which lead to the new influx of air. The larger the available space in the sleeping bag, the greater the convection. In sleeping bags with a larger volume, more energy is needed to heat them.

Wind Chill – The Cooling Effect of Wind

With every breath you take, your body releases moisture and heat into the air. This effect is increased at cold temperatures and wind increases this effect many times over. It influences the microclimate in your tent or around your sleeping bag, increasing the loss of body heat and causing you to cool down faster. Therefore you should always choose a place as sheltered as possible from the wind for sleeping when it’s cold outside.

Evaporation

Sweating is a natural process for optimizing body temperature. Breathable materials ensure that the moisture created by this process doesn’t stay inside the sleeping bag but is transported outward. This maintains the sleeping bag’s thermal performance and keeps you from starting to feel cold and clammy inside of it.

Physiology

Of course, a sleeping bag doesn’t generate any heat itself, but only retains the heat generated by the person inside. How warm you actually feel in a bag is greatly affected by a variety of physiological factors: Important factors include body weight and height, but also gender and age as well as your current physical condition:
People with greater fat reserves and those with more muscle mass cool down more slowly than slimmer sleepers. Due to the insulating layer of fat or the heat generated during muscle activity, these types are less likely to feel cold. Women tend to feel cold quicker and more often than men because their body fat is distributed differently and their blood pressure is lower and therefore their blood circulation is slower. . You should also keep in mind that people’s metabolic rate and muscle mass tends to decrease with age, when means that they produce less heat.
People that are physically fit and often outdoors are less susceptible to cold. In addition, a rested person produces more heat than an exhausted one.