I'm no dream expert, but from what I've read, one cause of nightmares is anxiety, related to stressors from the past. If you've sabotaged your diet before, you might be afraid to do it again. But the good news is that you can often rewrite your dreams while you are awake. The technique is called imagery rehearsal treatment and it is a cognitive behavioral therapy approach. Start by revisiting the scenario where you sabotage your diet, but visualize a new ending with a different reaction. Picture yourself looking at the food, deciding if you love it, whether or not you're hungry, and if you want it now or later. Then, if you choose to eat, picture yourself eating it slowly, while seated and with enjoyment.  Also, during your waking hours, be sure to eat enough food to allow your body sleep soundly, and make sure that your nightmares aren't due to a medication side-effect or some organic reason.