How to resolve the light issues in apartments & hotels while travelling?
Contact the provider before, sleeping masks, has anything else been invented?
Sleeping - How to get rid of the light in the accommodation?
I doubt the provider will care if you find the room too light.
On the rare occasions that this has been an issue for me I've found a blanket tucked into the curtain rail works pretty well.
If the curtains don't close properly you can use bulldog clips (carried with you, obv) or possibly even the type of trouser hanger which has clips.
Some people take a roll of aluminium foil, wet the window & attach the foil. It should peel off with no issues.
If you're really desperate you can buy portable blackout blinds/material.
Thanks, portable blinds sound like an option, looking into that now.
I contact them before to check if they have thick curtains or blackout blinds and immediately I'm a difficult guest for some, straight out 
Sleep in the bathroom
For years I was choosing cheap places to stay in. Curtains did not offer much protection from sunshine or blinking sign advertising. In my travel bag I carry a thin blanket or thick sheet, whatever you want to call it. (I use this for when my flights are cancelled and sleeping in airports.) This sheet works pretty good draped over the curtain rod to darken the room.
Like on planes, trains and buses - a sleep mask can work too! Sometimes you get them free when flying first or business class. :
Quoting greatgrandmaR
Sleep in the bathroom
Or under the bed
Quoting karazyal
For years I was choosing cheap places to stay in. Curtains did not offer much protection from sunshine or blinking sign advertising. In my travel bag I carry a thin blanket or thick sheet, whatever you want to call it. (I use this for when my flights are cancelled and sleeping in airports.) This sheet works pretty good draped over the curtain rod to darken the room.
Like on planes, trains and buses - a sleep mask can work too! Sometimes you get them free when flying first or business class. :
Thanks, it's the accommodation I struggle with the most, though maybe I missed that aha moment somewhere. Blankie it is