Measuring and stating the temperature span of a sleeping bag is very complex. Basically we generate heat through our metabolism – and in a sleeping bag we lose heat through conduction to the ground, convection via air circulation, respiration from the air we exhale, evaporation of sweat and radiation of infra red heat, where the two former are the most important. For comfortable sleep we must have heat balance, which is the difference between the heat generated and the heat lost through the above listed factors.
All these factors again depend on the metabolic heat output which is a very complex issue involving body fat index, age and gender – but also experience and lifestyle influences the sense of warmth. Younger persons generally produce more body heat than older ones, a fat person often feels warmer than a thin person, experienced lose less warmth than rookies – and women are generally colder than men.
The EN 13537 standard
All Yeti sleeping bags are tested according to the EN 13537 standard, which defines the temperature by which you are guaranteed a full nights comfortable sleep without waking up from being cold.
This temperature defines the maximum at which a man can sleep without sweating intensively defined with the ventilation zippers open, arms outside the bag and the hood open. It is also the lower limit for the average woman and according to EN 13537 this value is 5º C higher than for a standard man
This temperature defines the lowest temperature at which a person has a good nights sleep without waking from being cold. It is also the lower limit for the average man.
This temperature indicates the lower limit for risk of health damage and is the lowest temperature at which the sleeping bag protects the user from hypothermia.