Sleeping Bag Temperature Ratings
Most manufacturers will provide a temperature rating for their sleeping bags, and will generally show a Comfort, Limit and Extreme Rating. The following best describes each one.
Comfort - This is where you will most likely get a comfortable night sleep and feel warm.
Limit - This temperature is where you may find a slight chill if the temperature drops below the comfort level, but will generally sleep ok.
Extreme - This is purely a survival limit and there for information on what you could use it in case of emergency.
Remember everyone sleeps differently, and can vary depending on how much food and drink you may of had [especially alcohol]. It can also vary between male and female. If you know you are a colder sleeper then look at a lower comfort rating.
Another tip is that if you think wearing multiple clothing layers will help. This may not always be the case. Sleeping bags warm up from your own body heat and keeping it inside. If you are wearing lots of layers of clothing then your body heat will warm up those layers first and not the sleeping bag, Generally a thermal base layer is ok but don't go putting on lots of clothes.