Hey just-n, the pupil tells if there are major stresses in the body. If it is small, it means the person is experiencing extreme nervous tension. And when the pupil is very large it can mean that there is nerve exhaustion. Have you ever watched a doctor shine a light into your eyes and then pull the light away. The rate that the pupil responds to light is important. Sometimes drugs/RX can cause changes. (refreshed 48124)