A sleeping bag does not generate warmth but retains the warmth produced by the body. If you feel tired and cold and slip into a cold and damp sleeping bag, you are likely to feel cold, however good it is! These temperatures therefore depend on the person's resistance to the cold (bulk, fatigue, etc.), their equipment (mattress that insulates from the cold, etc.), their clothing (naked, underwear, etc.) and the weather conditions (humidity, wind, etc.).