Cortisol testing is a difficult process to accurately evaluate cortisol levels, which is why testing cortisol over a period of several days is the most reliable way of getting accurate results. While cortisol levels do rise and fall throughout the day, they also differ according to how stressed someone currently is. By taking cortisol levels at different times of day for multiple days in succession, it gives a more thorough insight into cortisol production, leading to better analysis of any issu