I had wondered what the relationship between intermittent fasting and cortisol levels were. I had asked before and got no replies. So I decided to search for myself. I found this paper which has some data.
The cortisol levels vary daily. It seems though that after about 18 hours of fasting the level rises above the maximum of the daily cycle. It doesn’t answer everything but it is a start.
Cortisol levels increase during fasting. The stress researcher Selye already described in the 1950s that fasting has a similarly good therapeutic effect as cortisone—without the negative side effects.
The rise in cortisol due to fasting has been confirmed in studies. Fasting has proven effective for chronic pain disorders and chronic inflammatory diseases.
Nakamura Y, Walker BR, Ikuta T (2016). A systematic review and meta-analysis show an acute increase in plasma cortisol levels after fasting, but not after mild calorie restriction. Stress, 19(2), 151–157. doi: 10.3109/102538. PMID: 26586092