Don't complain and make the poor window seat people who paid to look out the window at the beautiful landforms and admire the engineering of the aircraft's powerplant/wing suffer and sit in darkness when the problem can be solved so easily:
As others have pointed out, people wishing to sleep can use masks (and ear plugs) - I know I do on long haul.
I certainly wouldn't expect another passenger to miss out on a fab view, because I need to rest.
That would be extremely selfish behaviour IMHO
In that case you're not really very much in need of rest. Or the other pax is the one being selfish.
OK. I give in, let's reserve all the window seats for six year olds who have never seen the ground (or flown) before.