I also have a Western Mountaineering Ultralite, which is my 3-season bag. If it's a warm night, I may start the night on top of the bag or use it completely unzipped as a quilt. Usually by 2-3 am I start zipping it up. At higher elevations it's usually completely zipped up by morning.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey