Cortisol is at its highest in the morning (the cortisol spike is what allows you to have energy when you wake-up). If you don’t eat, it will stay elevated and even increase.
And if you couple it with cardio, which also tends to jack up cortisol output, you’ll end up with a sky-high cortisol level, which is one of the best ways to lose muscle.
Not only that, if it gets high enough you’ll actually have a hard time bringing it down during the day (especially when in a caloric deficit). You end up spending the whole day in a muscle-wasting state!
Cortisol wants to break down your muscle and increase body fat, the very things that you hope to avoid.
cortisol is a primary anti-stress hormone. Without cortisol, you would go into shock and die if exposed to trauma.
Cortisol also plays an important role post-exercise, by helping to supply fat in order to power muscle-protein-synthesis reactions. So cortisol is definitely not all bad