Automatic Headlight Sensor Problem

Does anyone know the "complete" conditions under which the headlights are suppose to go on. My headlights have been going on at odd times, so I had it adjust to "not sensitive" - apparently there are only 2 setting "sensitive" or "not sensitive".

However, even after having done this excercise, I find the lights go on when I drive under bridges, and remains on for the remainder of the journey - in bright sun light I may add. Sometimes it switches on when I use automatic kick over.

I am begining to conclude that I have a faulty unit. HELP>