A Five-Ounce Sleeping Bag
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.