Leisuremiles, when that happens to me (I like the cabin dark too), and someone is sleeping adjacent to an open window shade, usually
A) A(n) FA will reach over and close the shade without comment, or
B) I will do what the FA should have done in (A)

Thus far I have not had any problems with this approach. Of course, if it was a pesky FT'er sitting near the window... well, that would be another story