[QUOTE=ARGENT BRICK;866464]Let's take this a step farther. We will use a +40 degree rated sleeping bag as an example. Normally, a bag rated at plus forty degrees will allow you to survive in a forty degree environment, but you will not be happy or comfortable. Most experienced adventurers will carry a bag that is rated colder than their conditions call for.[/QUOTE]
Experience is the key, not hard and fast rules about ratings. I sleep very warm. I am quite comfortable in my +45 degree bag at +45 (when in a tent on my insulated-with-down air mattress). That will be about the only time I bother zipping it up. My +25 bag is useless for me at +45 as I'm too hot and throw the bag off... then I'm too cold. Back and forth all night between too hot and too cold. I learned this the hard way which is why I now have the +45 bag.