Might it be that hominin populations outside of the tropics thrived in spite of the seasonal dip in vitamin D and that emulating such a cycle would be suboptimal for health? Same might go for anything else.

I have lived 150 miles south of the Arctic Circle for over 10 years. This will be my first winter supplementing with Vit D. I never thought much about Vit D before discovering paleo, but i'm sure my levels were super low. Used to be I'd gain 15-20lbs in the winter and lose (most) of it the next summer. I've managed to lose over 50lbs since January eating paleo and exercising. It's tough to get motivated right now. Today's sunrise/sunset were 9:27am - 3:48pm. We lose 6-8 minutes a day until Dec 21st then start gaining. By then the sun is only up about 2 hours. Today's high temp was -15 and expecting -35 tonight...

Yeah but didn't a lot of traditional cultures, such as the inuit, Celts or Nordic people specifically eat vitamin D rich seafoods throughout the year to offset their dark seasons? Cod liver, seal blubber, etc?? If there was a dip, I doubt it was a significant one

I don't know the answer to your question, though I suspect that like most issues, it is highly individual. I know that for myself, if I were nomadic, I would be heading south for the winter to more warmth, more sun, and therefore, more vitamin D. From past personal experience, I know that I get pretty lethargic and depressed in these low light times if I don't supplement D and tan at a tanning bed(about ten minutes at a time three to four times per week). Perhaps this is a reflection of the fact that my ancestors were much more recently closer to the equator, and this would be a totally different scenario if my ancestors had been more northern/ mountain dwelling for millenia.

http://jackkruse.com/so-you-completed-the-leptin-rx-what-is-next/