....The worst possible scenario is you won't sleep much and you'll have to get up and move around or start your day early with some "night hiking". The coldest hours are usually 2-4 am. So if you've pulled up early in anticipation of the cold night to come... you may be able to sleep from 7 or 8 pm until 2 or 3 and simply start the day early. You've still gotten your 8 hours of sleep... and you avoided the part of the night where your bag is likely to "fail" so in effect you've simply corrected for the unexpected cold front by adjusting your sleep time and your bag worked fine. Going with the flow is the trick under most of UL philosophy really.
..... And many a "Speed hiker" will happily tell you of the wonders and life changing experiences to be found when you are on trail a few hours before dawn. It's exhilarating to mosey along for that first hour or two while you and your body wake up to the new day. And satisfying to take a siesta during the nastiest hours of the day's heat too if you feel you need a bit more rest. Being willing to walk outside the "9-5" schedule of many hikers opens up quite a few options..
Truer words there are not...