I second the layers thing. On a mid-May trip in Shenandoah last year, the wind suddenly whipped up on a cloudless night and dipped below freezing. I was in a Clark NA (totally battened down) with an ancient 40* down bag, an old thermarest ridge rest and wearing every bit of clothing I had. The layers made it work. Couldn't move all that well, but I slept warm enough (and I've had far colder nights in a tent).