To get a better picture of cortisol, a second test is performed around 4pm. If THAT one is elevated, then there are problems.

Also a dexamethasone suppression test is another way to test for cortisol problems.

The steroid is given the night before and the test performed the next morning. In a normal person the drug will suppress the cortisol you would normally make and the value will result low.
If cortisol is high in spite of the drug administered,then there is a problem (Cushing's is suspected for example with this test if cortisol remains high)
http://www.nlm.nih.gov/medlineplus/e...cle/003694.htm

Just taking a morning cortisol doesn't reveal much, as it is typically elevated for everybody on a typical day (active) night (sleeping) cycle. (night shift workers may show alterations in
levels because their sleep cycle is different).

Cortisol levels rise after breakfast daily. This is a normal function for the adrenal glands, to get you going. Cortisol tends to rise with eating at other times, but less dramatically so. It has been postulated by some researchers to be a factor in food addiction.
I find myself, that my arthritis pain is always more obvious when I am really hungry or have skipped a meal.

Test ranges are only statistical estimates of "normal". They are not absolute, and people will fall outside ranges and that may be "normal" for you. Ranges are not cast in stone, and are only best guess ESTIMATES. Then there are calibration issues at the lab level, and so then things may not be 100% accurate or reliable.