Id bet that the 18 seconds is related to the amount of time between when people naturally shift on their feet/comfort pocket... Meaning that, if you notice the

Id bet that the 18 seconds is related to the amount of time between when people naturally shift on their feet/comfort pocket... Meaning that, if you notice the frequency by which people shift their weight and move -- its likely in that window, assuming other externalities (bathroom, heat, thirst, anger, etc -- the physiological cycle of body comfort may be in that area of 18 secs)