well, that's just the thing. a nice -15C or so rated pile of down, and a good overbag or larger synthetic bag on the outside works great for me. I do wear full fleece and a balaclava inside.

Or you could drop the cash for something fantasitc like a WM puma, however I've never decided to drop enough cash for one. A bigger synthetic bag over a good down bag works for me well into the -25 to -35 range. Gotta have a good two or three pads underneath you too tho.

But for me really, part of it is I did grow up camping in temps like that, and well, I know what it takes to have a reasonably comfortable night *for me* (and it matters a lot by the person) frankly, knowing that is half the darn battle to know you're gonna be able comfortable and enjoy yourself, and that even if you get cold, you can get warm.