Having the wrong sleeping bag leads to not much sleep at all. When choosing a bag, match the temperature rating of the bag to anticipated temperatures of the location you’ll be in. Keep in mind, not all rating systems are the same between manufacturers.
Consider the following when choosing a bag:
1. What are the lowest anticipated temperatures?
2. Sleeping surface? Snow? Rock? Scree?
3. What is your sleeping style - Do you sleep cold or hot?
4. Wet climate or dry?
"Prepare for the worst, hope for the best. That’s my philosophy. I always go with a bag that’s 5-10 degrees colder than the anticipated low temperatures. For me, the extra ounces are worth it for a little more warmth and a good night’s sleep. " Peter Whittaker
"I sleep cold in the mountains, so I tend to bring a bag rated colder than the forecasted low temp." - Solveig Waterfall