The reference values are based on that time, when there should be the physiological peak of cortisol. Hormones fluctuate a lot during the day, cortisol more than others, so a simple blood draw is usually not enough to get a diagnosis. Performing it at the right time helps a bit, knowing that it should be at its highest level of the day ( the lowest is around midnight )
Just curious, why 8am? Is it tied to when most people get up or does it have to do with circadian rhythms and such?