After shivering on many trips with a half bag - most recently on the Tahoe Rim Trail with El Presidente - I recently sprang for a super light sleeping bag. The Sea to Summit Spark II sleeping bag has a total weight of one pound, and as you can see is not much bigger than a water bottle. Good for nights that get down to 35 degrees, it should certainly be excellent in the Sierras, Rockies, or Alps during June-July-August, and maybe a few weeks in either side.