I wonder if European descent has something to do with this.
Northern Europe is very far north compared to other countries. For example, if you look at the globe, half of people in Canada live at the same lattitude as Croatia.
Europe is uniquely warm for its lattitude and it causes unique situation where we have strong growing season and very dark winters. If you are in agriculture, you would want to stay up for long in the winter to do some at home stuff.
The data seems kind of mixed on that. I would guess that the variant had little selective pressure - being grouchy and tired all the time isn't enough to kill people or prevent them from reproducing.
The variant is particularly frequent in Ashkenazi Jews (which says very little about any selective effects), but it seems to be more prevalent in African-descent people than Finns, and more prevalent in South Asians than Europeans. So it doesn't seem especially common in Europeans, or even particularly related to latitude/longitude.
> being grouchy and tired all the time isn't enough to kill people or prevent them from reproducing.
It may not be enough by itself, but it's easy to imagine it being the last straw so to speak. An alternative explanation could be that society needs a certain percent of night owls. That there is a niche in which it's beneficial.
It’s a huge mistake to go grasping for an adaptive explanation to every single mutation. The null hypothesis is that “this disorder doesn’t kill people before sexual maturity, or provide some reproductive gain” and there’s literally no evidence for anything else. You are speculating about a solution to something that isn’t actually a mystery.
And as I said in another comment, the disorder this mutation causes is not normal “sleep late, wake late” night owl syndrome.
If the variant is prevalent then I can only see two explanation: either it is(was at some point) adaptive, or there was some kind of founder effect - where a small group of people that just happened to have it and become hugely genetically successful for other reasons.
The thing is that it isn’t that prevalent! Please look at the link I posted with the data - almost every ethnic group listed has less than 1% prevalence. The group it is most prevalent in is Ashkenazi Jews (3.4%), who have an unusual history that discouraged cross-ethnic relationships for millennia, and who are prone to many recessive genetic disorders that would have otherwise been more diluted. These mutations are not so prevalent because Ashkenazi Jews needed them to survive European persecution: they are the consequence of said persecution.
There really does not have to be an adaptive explanation or an adaptive “coincidence” like you suggested with the founder effect. Please please please please look at the data.
A common misconception about natural selection is that it optimizes species. It’s really more about “this has bugs but it’s good enough to cut a release branch.”
Just want to mention this mistake. We are talking about a gene variant, not a trait. Phenotype inheritance is much more complicated than genotype inheritance.
It might be simpler explanation. Homo Sapiens evolved in Africa where there are no long winter nights and huge variations of daily cycle throughout the year. Also humans can do shit in sunlight only. Like hunt.
Then we moved to a different zone where there is longer nights and then we learned to do stuff that could be useful to be done "after hours". The pressure on good nights sleep immediately after dark lessened. Actually, people had to learn to work in the dark because in central and northern Europe there isn't much sunlight in the winter. Even in daytime there is usually overcast and no direct sunlight.
I'm sceptical to how much evolutionary pressure we can see evidence of going back just 100 000 years or so (at the point when we became truly social creatures, I think culture, passing on knowledge, technology and ideas became much more dominant for survival/procreation than mere genetics).
That said, I think the theory that you need someone awake to ward of nocturnal predators (be they human/homid or animal) has some merit. Being eaten beforepuberty does tend to cut a genetic line short.
Ed: a counter example of "modern" evolutionary pressure would be sicle anemia/malaria. A "technological" fix would be mosquito nets, relocation to a part of the world without malaria, etc.
Or eating koka/coffee to stay alert on late watch.
My favorite example of a (real) recent selective pressure is lactose tolerance, which is well-correlated with ancestry in regions that a) are prone to vitamin D deficiency (Northern Europe especially) and b) regions that have cows or sheep, regardless of sunlight. So in particular most of Africa is lactose-intolerant, but many pre-modern West African groups were pastoral herders and can digest lactose. It is a bit of a mystery how this happened: other lactose-intolerant groups still used milk to make yogurt and cheese with much less lactose, and it’s not clear why there would have been pressure for adults to drink raw milk - perhaps a water-borne disease caused a bottleneck?
There's a good book on the topic called "Why We Sleep" by Matthew Walker, a professor of neuroscience and psychology at Berkley. In it, it discusses how teens are biologically designed stay up much later than their parents would, with the theory that this is when they would have a chance to socialise and mate without the watchful eye of parents. Not only that, it reduces the time that a tribe would be open to attack or prededation - if everyone slept at the same time, there would be an 8 hour window of the day where humans would be incredibly vunerable. Presumably it would be favourable to have a few adults in the tribe to have a similar night-owl streak, to keep the tribe secure throughout the night.
The book really pushes the narative that making kids, and especially young teens wake up so early just to fit into an adults daily routinee is akin to child abuse, as it stunts their growth in ways - physically and mentally - that cannot be achieved by any means except through sleep deprivation.
I live in Poland and I spent my young years on a farm. In the winter you barely see any sunlight. There is nothing to do outside but plenty to do inside as you take care for the animals and do other stuff that you don't have time for in the summer.
I assume this has been the same for pretty much everybody in Europe for thousands of years except for the last couple of decades.
The link I posted is the data you are looking for. In particular it days that the DPSD mutation is more common in Africans than in Finnish people, and more common in South Asians than Europeans. The lengths of days and nights seems to have nothing to do with the prevalence of the mutation.
So these “evolutionary adaptation” explanations are a) not supported by the data and b) unnecessary to begin with!
A simple explanations: the genetic difference that regulates the biological clock is tiny, which makes variations common, because on the perspective of millions of years such variance is useful, as it increases adaptability and thereby surviveability of the species.
Massive increase in hunters during the day? Small mammal ancestor of humanity becomes a creature of the night within few generations. Sudden ice age, night activity is energy expensive, so curl up, sleep, and search food during day is the best strategy? Adapting quickly to such changes is a benefit. To get that benefit the mutation must be common.
I don't think that there are benefits in stay up longer in winter, more like the opposite. It's cold and dark. Standard adaptation is actually to sleep for longer in order to conserve energy.
Staying up longer in summer may be beneficial, but this does not seem to be the effect of this mutation.
Northern Europe is very far north compared to other countries. For example, if you look at the globe, half of people in Canada live at the same lattitude as Croatia.
Europe is uniquely warm for its lattitude and it causes unique situation where we have strong growing season and very dark winters. If you are in agriculture, you would want to stay up for long in the winter to do some at home stuff.