We spent 2 weeks traveling Alaska staying in a variety (5) hotels, B & B'b, cabins etc. IT NEVER GETS DARK THERE! Only one place had dark window coverings, most had none so that you can see the view I guess. You would think they'd know we are tourist, tourist that come from places that actually have "night time".
My advice...pack different types of "pins" so you can pin up towels or anything else you can find to cover windows. We also pack those night masks to wear and that helped.