A lot of them have very strict routines about going inside and using super-dark curtains at particular times. I've gotten to email some people above 60N and it is just what they do.
In the old Aschoff "bunker" studies where people allowed to keep the lights on as long as they wished, they concluded the human clock was 25.1 hours. We know this isn't true if you provide a proper light-dark cycle, it's much closer to 24. But I treat this old work as a cautionary tale - we can be 25.1 hour animals if we leave the lights on all the time.
I grew up at 66N (Northern Iceland), for what it's worth blackout curtains are rare to nonexistent there in my experience. Sometimes bedroom curtains don't even cover the whole window, and during parts of the year it's daylight outside 24/7.
When I've traveled there with foreigners who grew up at more southern latitudes with year-round "normal" day/night cycles I've sometimes had to tape black trash bags to the bedroom windows for their benefit.
What do the locals do? We simply get used to sleeping in daylight. One way of doing that is getting used to sleeping on your back with one of your arms across across your face and over your eyes (such that your elbow is between your eyes/on your forehead and the back of your palm touches or is near the opposite shoulder).
I still sleep like that out of habit at more southern latitudes out of habit, and a quick poll among some fellow Icelandic expats where I live when this came up the other day revealed that they do too.
In Spain we have lots of problems and room for improvement in many issues, but one thing I can be proud of is that the vast majority of homes have blinds that close completely and get zero light in. I often wonder why it isn't like this elsewhere, and especially in northern places where it would be especially convenient (I get that it's possible to adapt, but wouldn't it be nicer to be able to sleep in any position?)
Of course, the problem with this is that I'm totally spoiled and when I travel abroad (1), I can almost never sleep well. Hotel rooms in most countries have blackout curtains but if you're used to Spanish blinds, even those let a lot of light in from the edges. And don't get me started about LEDs...
(1) PS: Now that I come to think of it, I don't sleep well when I travel within Spain either... curiously, people's homes have blinds but hotels typically don't.
I'm assuming you're talking about the exterior metal curtains common to Spanish houses (persiana con cajor exterior [1]).
Those are only incidentally to block out light, they're mainly for thermal management. You put them down in the middle of the day to keep hot air and direct sunlight out.
A system like that is completely unsuitable in places that get snow. If you installed it on a house in Iceland it would at best freeze solid, and more likely explode as snow slush made it into the joints and froze solid. You don't even need to go outside of Spain to see that, go up to Monachil in Granada and see that no house in the area uses that system.
Of course you could install blinds like that behind double-glazed glass, but at that point you can just buy normal blackout curtains. Since you don't need it for thermal management (any amount of external heat making into the house being a luxury) there's no point in installing it on every window in the house.
So then for practical purposes you're left with only installing blackout curtains in the bedroom, but if you live that far north you're going to need to just get used to sleeping in the light anyway. What are you going to do if you go on a camping trip where you sleep in a tent and forget your blackout eye mask, just not sleep?
As far as I know, they are for both purposes. In fact, most of them can be lowered enough to provide thermal insulation but still let light through (they have small horizontal holes for that purpose), but if they are lowered more, the holes close and they block light. So it's definitely a planned feature, not a side effect.
In most of Spain they are installed outside of the window glass, but in the north they are installed inside, so they are protected from cold and wind. As in this picture, for example (just found at random): https://www.inmoatuel.com/piso-exterior-de-90m2-reformado-a-...
But they still seal the window hole perfectly so they block light much better than blackout curtains. I suppose the problem with Monachil is that it's a cold place, but within Andalucía which is generally warm, so they don't have a tradition in the area of installing blinds inside as is the case, for example, in Galicia, where you can definitely see them even at high altitudes where snow is common.
When I go on a camping trip... indeed I just don't sleep well, I'm afraid :)
The picture you linked to is taken in A Coruña whose mean temperature is around 10 degrees higher[1] still than relatively mild[2] climates near the arctic circle.
I didn't mean to suggest they weren't also used to let light in. I've spent a lot of time in houses that have these installed, letting light throughout the day while keeping the house relatively cool is a constant balancing act. I meant that without the need for the thermal management you'd go for other sorts of systems, such as internal blinds.
I.e. I'd expect that in places like A Coruña you'd use these for thermal management in the summer with the outer windows open, whereas in Iceland there's maybe 1-3 days of the year where the outside temperature is anywhere near the inside temperature.
I live at 59.3 and dark curtains are not too common, most house I saw have some kind of shades or fluffy curtains, both don't block light
> they concluded the human clock was 25.1 hours
You keep quoting questionable or out dated research, the 25 hours figure seems to be wrong [0]
> Early research into circadian rhythms suggested that most people preferred a day closer to 25 hours when isolated from external stimuli like daylight and timekeeping. However, this research was faulty because it failed to shield the participants from artificial light
