Under the direction of the hypothalamus and pituitary, and controlled by a negative feedback loop, the zona fasiculata of the adrenal cortex is stimulated by adrenocorticotrophic hormone to produce cortisol in response to stressors. The feedback loop is commonly referred to as the hypothalamicpituitary-adrenal (HPA) axis. Cortisol production is normally at its highest upon waking and declines steadily during the day, reaching its lowest point at bedtime. Cortisol has a wide range of effects on mind and body and interacts with the reproductive, thyroid and immunological systems. As part of the stress response, it prepares the body for “fight or flight” and in doing so it can suppress the production of other hormones, including those involved with reproduction, and some immune functions. When cortisol levels remain high as a result of chronic exposure to stressors, this suppression of other systems is maintained for longer than normal and can result in susceptibility to infection, hypothyroidism, bone loss, infertility, and low libido. On the other hand, lower than normal cortisol levels can result from adrenal insufficiency or “burnout”, and are associated with decreasing attention span, fatigue, allergies, and blood sugar imbalances. Since both high and low cortisol levels are associated with multiple symptoms, diurnal cortisol testing can help identify the causes of complicated health issues. Saliva collection multiple times during the day is convenient for assessing diurnal variations in cortisol secretion. The ZRT reference range for cortisol levels is 3.7—9.5 ng/mL (morning); 1.2—3.0 ng/mL (noon); 0.6—1.9 ng/mL (evening); and 0.4—1.0 ng/mL (bedtime).