Q: When choosing a sleeping bag, does the degree level really matter or is it just a recommendation for comfort?
— Ethan, Springfield Township, New Jersey
A: If you were dressed in a base layer of clothing and zipped up in your bag on a winter sleeping pad in a climate-controlled room then, yes, the degree level should be completely accurate.
Testers with the International Organization for Standardization (ISO) rate sleeping bags by dressing up mannequins covered in heat sensors. Those ISO ratings are often reflected in the bag’s name, like the NEMO Forte 35 ($160, nemoequipment.com), which is rated to keep an average sleeper warm at 35 degrees Fahrenheit.
The outdoors is not a controlled environment, so look at degree levels more as guidelines. The weather, your type of tent, pad and clothing, and even how you sleep all play factors in keeping you warm at night.
The sleeping bag itself even plays a factor. The NEMO Forte, for example, includes a pair of zippers to vent body heat, making the bag more comfortable on milder nights.