As a person with ADHD and who noticed that whenever I have a few weeks off (so no schedule restrictions, so to speak) my sleep schedule seems to advance by almost an hour a day, this sounds like it might explain a few things.
Generally the endogenous clock is approximate (hence the word circadian, about a day). But that’s because the day length has been constantly lengthening since animals evolved. Too slowly for us to notice, but the proteins that handle clocking are fairly ancient (though there are additional copies in humans compared to a fruit fly, say), and they aren’t locked in to favor a rotation period for Earth because there hasn’t been a stable period over long stretches.
The clock entrains to the environment, though. The more the daylight brightness to night time darkness shift in intensity, the higher amplitude their rhythms are, generally. Other signals also entrain them, like social activity, and the gut clock is a whole other complexity (kinda downstream of the brain clock but it can feed right back and wreck the central oscillation).
Any number of these factors can cause circadian effects, from a flatter rhythm, changed period, entrainment mismatch, etc.
In rare cases when a circadian gene itself is mutated, then you can have the clock period stretched significantly, and then synchronizing to the 24 hour day becomes really hard.
Well, this is also interesting, because I was born in Ghana and spent the first four years of my life there, and since then have always had insomnia issues, first in the Netherlands and now Sweden. However, the one time I visited Ghana as an adult for a month I slept like a baby.
I actually had a gut feeling that the much quicker shift from day to nighttime darkness and back played a big part in it, but wasn't sure why that might be the case. I assumed it was psychological somehow, that on some subconscious level I felt more "at home" in Ghana's natural environment because it's what my brain was "calibrated" on in the first years of its life. Would be interesting if that actually is the case in a circadian rhythm sense.
> That’s the endogenous period, that really only comes out as is in total darkness over many days.
And speaking of Sweden, I wouldn't be surprised if the short winter days probably do give enough "leeway" to approximate that effect.
As for Sweden: the low light in winter definitely impacts the clocks function. But still not constant darkness, since even a bit of sunlight is some signal to the clock, and it’ll entrain.