Plasma melatonin testing is a quantitative measurement of the level of melatonin hormone in blood plasma. This hormone is secreted primarily by the pineal gland in the brain in response to darkness and is known as the "darkness hormone" due to its crucial role in regulating the circadian rhythm (biological clock) and inducing sleep. It is best measured at specific times (nighttime) or through a series of measurements to plot its secretion over a 24-hour period.