Comfort temperature in sleeping bags refers to the lowest temperature a bag is intended to keep the user warm and comfortable, without the need for any additional blankets.
When a bag is designed as a “15-degree bag” it means that the average sleeper would remain warm and toasty inside the bag unless the air temperature drops lower than 15 degrees. These sleeping bag temperature specifications assume that the sleeper is wearing long clothes and is using a sleeping pad underneath.
However, age, gender, and metabolic rate vary from person to person and this could affect how one sleeper perceives the comfort level of the bag he or she is using. Thus, use these ratings only as a guide because they are not a guarantee.
Most bags have a rating of +15 degrees and +50 degrees Fahrenheit. If you are unsure which bag to bring to a trip, always select a sleeping bag with a lower temperature than your destination. It is advisable to err on the side of caution; if you’re too cold, you are bound for a terribly restless night, whereas if the air temperature of your destination is higher, you could always unzip to allow air circulation inside your bag.