Cortisol increases to a max while you sleep and then slowly drops off over the course of the day, known as a Diurnal Cycle.
Let me say, that I am by no means an expert in this, I just had low cortisol and spent months working through natural ways to raise my cortisol to healthy levels. The number I kept seeing was around 16 as a target for am-cortisol. The import thing however is that your body follows a normal diurnal cycle. Some people experience sleep disturbances because they get cortisol spikes in the middle of the night.
It sounds like you are more concerned about keeping your cortisol levels lower, but I would caution you to aim for healthy cortisol levels. The 4 point saliva test is the gold standard here for most integrative medicine docs, and you can get this through ZRT labs and many others.
Here is a good study that talks about the impact of low cortisol levels and anti-social behavior, and it also has some good general info about cortisol cycles etc.http://www.nature.com/npp/journal/v32/n7/full/1301289a.html
Here is a good synopsis of the stages of Adrenal Fatiguehttp://www.thyroidhelp.org/forums/showthread.php?1826-Adrenal-Fatigue-Seven-Stages-Stage-1
If you feel stressed out and your levels have been creeping up, it could be that you are overworking your adrenals and you might consider taking action to manage stress better.
Mind you, most traditional docs don't recognize adrenal fatigue and in my opinion that is because they are right that it is often the result of some other condition going on in someone's life, so not in itself a diagnosable disease like Addison's is when the Adrenal's don't respond to ACTH. The problem with this however is that traditional docs still don't treat the whole patient and if there tests show normal results they move on looking for a disease to treat. This often leaves patients feeling like hypochondriacs during the process. For me the big question is what was the nucleus of the stress's that lead to the low functioning adrenal condition, is it psychological or physiological. Did it start out as physiological and then become complicated due to an untreated condition causing a psychological complication. Our Western medicine system is not equipped to unravel the complexity of these diagnoses and therefore it requires an open minded doctor and a patient committed to his/her overall health. In my opinion.
STTM also has a good page on Adrenal Fatiguehttp://www.stopthethyroidmadness.com/adrenal-info/
Hope this helps, let me know if I missed what you were asking.