okay i brush my teeth very well everyday an night.

but..every morning when i wake up an look in the mirror, theres always this line across my two top front teeth, about half way down. Above the line seems to be discolored (kind of dirty looking) and below it is white (clean)...why is this? i tried white strips, helped a little but.. that line halfway down is still there... what do i do to get rid of this? i want clean white teeth..