The fasting scientists I’ve read and interviewed (I’m a reporter) usually say it’s OK to break your habitual eating times now and then, but it’s best to keep them as consistent as possible, especially the start time. The reason is that many of the body’s circadian rhythms are set by what times you eat each day. If you’re constantly changing that timing, you’re throwing your body into what amounts to metabolic jet lag from one day to the next. Satchin Panda of the Salk Institute has probably done the most influential work on this. Since you’re a shift worker, you may also be interested in his related findings on the effects of shift work. See his book The Circadian Code.
Increasingly, research is also finding that the eating window that syncs best with our circadian rhythms is an earlier one than what many of us are used to—say, starting your eating a couple of hours after you wake in the morning and finishing six or eight hours later. Studies are finding that we’re hardwired to process nutrients most efficiently in the morning and early afternoon, and eating later dings us up at a cellular level. I know that sucks for shift workers, but the studies behind these findings are pretty solid.
Have you actually tried to do more physical activity while fasting? I used to think that I had to eat before I went to the gym or did something else physical, but as it turns out, I do great when I’ve already been fasting for 16+ hours when I go to work out. So if you haven’t tried it, I would encourage you to do so and see how you do. You can always bring a snack in your bag in case it gets to be too much.