Simple is not always better. Case in point: the standardized sleeping bag temperature rating system—called EN, for European Norm 13537. At first glance, it’s more complicated than the previous temp ratings, which provided a single number, say 20°F. But our testers like EN; they think the widespread use of the system has led to more accurate ratings overall.
The key improvement: EN ratings provide a range to address the disparity in sleeping comfort between genders (women typically sleep colder) and from individual to individual based on myriad factors like body fat, tent, sleeping pad, clothing, hydration, and metabolism. EN tags list three specific temps: Comfort (the lowest comfortable sleeping temp for an adult woman), Lower Limit (the lowest comfortable temp for an adult man), and Extreme (not comfortable for anyone to sleep, but warm enough to prevent fatal hypothermia in a woman for six hours). Note: EN test dummies use a standard pad and wear baselayers and a hat.