Measurement of long-term endogenous production of cortisol can be done by taking urine, saliva or serum samples, while each of these measurements has advantages and limitations. All these methods require multiple samples to provide an assessment of cortisol levels over prolonged periods of time and it is important to note that those measures reflect acutely circulating cortisol level or integrated cortisol secretion over the examined sampling period. Another aspect is that acute cortisol level fluctuates markedly depending on many factors such as circadian rhythmicity, acute stress, cigarettes smoking, alcohol, or exercising.