Originally Posted by Seat 2A

Typically it would be in the morning on an eastbound trans-Atlantic or trans-Pacific flight where people are sleeping. During those times it is considered rude to open one's shade.

On the other hand, I've often flown westbound between Europe and the States or the States and Asia where the flights usually depart midday and on those flights most people are awake anyway. If they want their window shades down, it's usually so they can see their laptop or seatback screens better. I'm a lot more likely to leave my window shade up during those times. After all, the windows are there so you can look outside and what better and less obtrusive time to do so than the middle of the day when most people are awake?