Normal and abnormal diurnal cortisol secretion patterns are identified from human saliva samples by an in vitro method. First and second saliva samples are taken from one human individual at first and second predetermined times during the same day. A first cortisol concentration is determined in the first saliva sample, and a second cortisol concentration is determined in the second saliva sample. An abnormal secretion pattern is then compared to a normal secretion pattern with the help of a fuzzy logic algorithm. A function of the hypothalamic-pituitary-adrenal (HPA) axis is then determined for the human individual. Optionally, a testosterone level is determined from one of the samples and is used in combination with the cortisol concentrations to provide a redefined determination.