This is the payoff when doing IF. Cortisol is not to be shunned per se, especially in the AM. The whole rationale behind an IF plan like the Warrior or Renegade Diet is that you are in an advanced fat burning state part of the day, and in an anabolic mode later. Folks will debate from now until doomsday whether this is possible but unless you are willing to step into the broscience lab and try it yourself you will never really know. Of course, if you are hammering down coffee, stressing at work, and are sleep deprived, etc, your cortisol levels are going to soar and a fasting diet will certainly become non-productive (as a normal one would too).
I can only state from personal experience that doing an IF programme is the nearest I have come to a build and burn state, i.e. where you are consistently losing body fat on a reduced calorie diet, yet maintaining and, at times, gaining on lifts as well as workout performance, which for me is at least 5 sessions a week. Again, from personal experience, the peri-WO nutrition affect also appears enhanced when consumed later in the day on a near empty stomach.