As other mentioned, there are eye masks in the amenity kit for those who wish to sleep. I don't see a reason why shades should be closed.

I usually keep mine closed on overnight flights but I absolutely hate it when crews force 787 shades dark on a daytime flight. I had it happen on TK IST-PTY recently and it was pure hell, especially when one tries to fight jetlage on a westbound flight by staying awake.