Okay, it wasn't quite a sleeping bag. However, it was only five ounces, and it kept me warm as the temperature dropped to the low forties on the bank of the Manistee River in Michigan. The secret was the fifteen minutes we spent gathering dead, dry bracken ferns to build a two-foot thick mattress. We set the tent on that. Then, with all my clothes on, I was fine.
In fact, I've rarely slept as well camping as I did that night. As I said, it wasn't a sleeping bag, but actually a sleeping bag liner. I bought it from Campmor, but I've since sewn a simple one of bargain-bin nylon material ($1/yard) obtained at Walmart. If it's possible to stay warm with a light sleeping bag liner in autumn, at a few degrees above freezing, this strategy should work well for summer nights in the sixties.
How To Use A Liner As A Sleeping Bag
Be careful, of course, backpacking with just a liner for a sleeping bag. It could be dangerous, or at least uncomfortable enough to ruin your trip. Experiment near home, and know yourself and your enviroment.
Learn a few tricks. If it isn't too humid you can breath in your bag, and you will be much warmer. Most backpackers will tell you not to do this, because you'll be damp in the morning, but in a dry enviroment you'll be fine once you hit the trail. Just dry the liner out during a break.
Another trick is to use a mattress of dried plants. Try dead leaves, palm fronds, grass, cattail leaves, some tree barks, etc. A mattress of this sort insulates you from the ground, which normally takes away much of your body heat. Scatter the leaves in the morning so they won't smother the plants underneath.
Some other tricks to try: Hot tea before going to sleep. . . Exercise a bit. . . Cover yourself with extra clothes. . . Elevate your feet slightly. . . Go to sleep earlier or later. Experiment to see what works best for you.
Also, go to bed warm. If you're warm when you get into your sleeping bag, you're more likely to stay warm through the night. It's difficult to warm up, especially in a thin bag, if you start out shivering.
I'm not recommending backpacking with only a sleeping bag liner, but it is an option. I've gone out with nothing more than a bivy sack in my jacket pocket, but I'm not recommending that either. This is just to present all the possible options for the ultralight backpacker.
Steve Gillman is a long-time advocate of lightweight backpacking. His tips, photos and stories can be found at The Ultralight Backpacking Site : http://www.The-Ultralight-Site.com