Perhaps I'm a cold sleeper--although I once did a bivy in the back of my car at 35F with nothing but my office clothes and some newspaper for insulation--but I've comfortably slept in down bags rated at 0F and below at nightime temps of 60F and above.

Back in the early '70s, we were teens and could only afford one bag that would have to do for both winters in the Sierra (once above 13,000 feet) and also summers in Joshua Tree. There were no temperature ratings then, but we all had bags with about three pounds of down and a loft of 7 inches or so--the equivalent of a modern 0 to 10 degree-rated bag. And you know what, we did just fine--I worked at a summer camp in the high desert of San Diego County, and for 70 straight nights slept in the same bag that I used for a winter climb of Whitney.

Down is much more breathable than synthetic insulation--in winter I sleep in my house in a Marmot OF bag, and in the summer (when temperatures can often be 85F at midnight) in a 30F Feathered Friends bag. I've tried the same thing with an ID Emperor (primaloft, rated at 0F) and there was plenty of sweat.

A Feathered Friends Snow Bunting, WM Super Antelope, or Marmot Lithium would all be relatively lightweight do all bags for 0F up to whatever you can stand.