A trick that I learned from camping in Yellowstone at 8,000 feet was to combine synthetic sleeping bags with my down sleeping bag. This is for car camping not backcountry camping, I use the synthetic bag as a thermal break from the cold ground and then sleep in my down sleeping bag. I make an exception to buying the best camping gear that I can afford when it comes to synthetic sleeping bags, the inexpensive kind that you can find at any camping store, are just fine when used as a thermal break. Unlike computers and digital cameras, good camping gear can last you a lifetime. I still use the down sleeping bag that I bought in 1971. The hiking boots that I bought that year lasted for 15 years. The down jacket that I bought that year now looks a little ratty and I really should send it in to be reconditioned but I've become emotionally attached to it. It is important to understand that the lowest campground in the park is at 3,000 feet. Many Glacier Campground is at 4,800 feet. Nights in the mountains can be very cold even in August. Not only can the nights be cold but it can snow 12 months a year in Glacier National Park. About 10 years ago it snowed 4 feet overnight on Memorial Day weekend in East Glacier, Montana. June 20, 2008 it snowed two feet.