When I taught mountaineering at NOLS, we required every student to bivouac with thier peak summit gear (in a controled condition, so if they got really cold we brought them their sleeping bags). You do not sleep, but you learn what clothing it takes to survive. Taking all your gear to the summit is not practical. There are special light weight heat blankets (survival blankets)you can use. Why do people "guess" at what they need instead of actaully practice bivouacking? I am the "Queen of Bivouac" - spent over a dozen nights caught up on moutain peaks.

Now every time I do a day hike from my backpack base, I always assume I will have to spend the night out and take enough clothing and food. In a backpacking situation, the more critical item is always keeping track of your last water source. I also am always looking at surroundings and judging them as suitable bivouac sites. Like "wouldn't it be cozy under that tree?"