I'm not totally dissing the diet connection. A person who doesn't get any protein during the day, has a stressful job and survives on stimulants, sweets, junk food, is asking to "crash" by the end of the work day (been there done that). So then you eat a high protein meal and take a nap allowing your adrenals to recover. Now your adrenals can catch up with the request for production coming from the brain/pituitary. When you try to go to sleep you have high-norm cortisol and struggle to get to sleep.
However, the body naturally makes much, much more cortisol in the morning than in the evening!