As mentioned, sleep layers. If you aren't planning on further extending your seasons, a true 15 degree bag or quilt should be plenty. I use an EMS Mountain Light 15 down to 0 F by wearing my down puffy and a hat. Considering the morning is going to be cold when you arise, and temps will drop in the evening, you may consider factoring in your cold weather gear as sleepwear. You'll need it, might as well use it.