A word of caution on the temperature ratings specified by US retailers.

Here is an example of the specification for a high spec synthetic Bag - Thermarest Centari™ bag.

http://www.cascadedesigns.com/therm-a-rest/sleeping-bags/centari-0-sleeping-bag/product

The EN Comfort rating is -5 C / 23 F

Yet the Rating given by most retailers regard it as a -18 C / 0F bag.

If you are going to go out in dangerously cold temperatures please go by the EN Comfort rating to give yourself a safety margin and insure the insulation under your sleeping bag has a R rating of at least 4.5 when using a synthetic sleeping bag.