I suggest that you research the average temperatures that time of year for the location where you are going, and assumed that it will be at least 5-10 degrees colder. Get a sleeping bag rated for the temp, plus two mylar "survival blankets". These are thin sheets of plastic with a silver coating. They reflect heat amazingly well. You can put one under your bag and one over it. I did this on a camping trip last week and was nice and warm even though the temp dropped two degrees below the rating of my sleeping bag, and I was sleeping in a car instead of a tent, which gets a lot colder.
Another tip: Instead of wearing layers of clothing inside your sleeping bag the way you do when you're walking or paddling around, expose as much skin as possible, given whatever social setting you've got. This way, you'll heat up the small air pocket inside the sleeping bag with your body heat and form a nice warm bubble around you. Eskimos used to sleep nude under fur blankets. I wear a t-shirt and shorts to sleep when I camp at temps right around freezing or a bit below. (I personally don't camp any colder, but the same principle applies. You just need a lower-rated sleeping bag.)
