This graph shows the circadian rhythm (i.e., daily fluctuations) exhibited by human core body temperature under resting conditions. Although it is commonly said that human core body temperature is 37 °C (98.6 °F), during any given day, core body temperature fluctuates around this value. During the early morning hours, core body temperature is about 0.5 degree below 37 °C, and in the late afternoon, core body temperature is about 0.5 degree above 37 °C. Therefore, the core body temperature is not constant at 37 °C, but rather it fluctuates around this set point. The set point is simply an average of all the points that fall within the normal range. Therefore, 37 °C simply represents the statistical mean of all the temperature measurements made from a large number of "healthy" individuals in a population. The value of the set point is an intrinsic property of the system, and it is generally independent of the changes in the internal and external environments. In some cases, the set point can be reset. For example, fever resets the set point for core body temperature to a higher value. It is important to note that most physiological parameters show a similar pattern of fluctuating above and below a set point. This ensures that each physiological parameter is maintained within a narrow window, with minor fluctuations above and below its set point. In most cases, physiological parameters are maintained close to the set point by negative feedback loops, and the collective action of these negative feedback loops leads to homeostasis and maintenance of health and life.