What you need to know about temperatures
A sleeping bag does not produce heat, but retains the heat produced by the body. If you are tired and cold and you slip inside a cold, damp sleeping bag, it is very likely that you will feel cold no matter how good your sleeping bag is! Your thermal comfort will therefore depend not only on climatic conditions (humidity, wind, etc.) but also on your sensitivity to cold (fatigue, etc.), your equipment (insulating mattress, etc.) and your clothing (naked, underwear, etc.)