Intermittent Fasting and Cortisol levels

Hi Brooke,
I heard that when we do intermittent fasting our cortisol goes up during the whole fasting time. Is that true? I am often doing 18-21 hour fasts and feel well overall, at some moments I feel very energized. I am a bit concerned because I know that having cortisol in our bodies for long stretches at time is not healthy for our bodies. I feel that intermittent fasting is really helping me with releasing weight, physical energy, mental clarity and building resilience so I don’t want to stop it or shorten it. Would appreciate your thoughts on this.