I can see why people would think this idea works, it has been 8 – 12 hours since you last ate, your body still is actively working and using calories and no food is going in to provide more energy, so in this slowed metabolic fasted state body fat stores can be used in place of the glycogen. Then why not just stay fasted through the morning? There is a adage that fat burns in a carbohydrate flame. Which is really saying that the body needs glucose (Carb Source) to lead the fat burning? With less than suitable glucose accessible to keep the body up and running correctly. Exercise intensity won’t be maximized, which in turn won’t maximize calories burned.