Earlier quoted context omitted.
I lived in Norrbotten for 10 years. Blackout curtains help, but it's not the same thing.
In Spain every house comes with 'Persian blinds' ( https://es.wikipedia.org/wiki/Persiana ) installed. They completely block the passage of light and are a perfectly adequate solution to this problem. It's a mystery to me why no other country in Europe seems to know or care. So much so that there's no English version of the above Wikipedia page.
Blackout curtains themselves do a good job, but have a lot of side bleed. Blinds even out the problem by.. also adding side bleed between each of the sheets.