I have posted this before somewhere but I use a good old fashioned hot water bottle, much better that a regular water bottle as they have seals for boiling hot water, you don't ever want them to leak. They are much better than an electric blanket as they do not reqire continual power and are much warmer. I grew up with these in the north of the UK where they are still common to this day as Brits don't like eleectric blankets.
http://www.vermontcountrystore.com/...=1&keyword=hot+water+bottle&searchid=inceptor
Fill it with boiling water 30 minutes before bed and put it in its cozy and then put it in your bag, leave it in there when you get in, becareful because it will be red hot (hence the need for the cozy). It'll still be warm in the morning and no aux power required, in those temps you do not want to rely on a battery.
Along similar lines, put a thermos full of water in your bag and you can use it to make coffee in the morning without ever getting out of the bag. I do this all the time and its awesome! If you use a hot water bottle then keep them together by puting the thermos in the cosy with the hot water bottle.
Rob