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! These temperatures therefore depend on the person's resistance to cold (bulk, fatigue, etc.), equipment (mattress with insulation, etc.), clothing (naked, underwear, etc.) and weather conditions (humidity, wind, etc.).