In the USA the cultural atmosphere slows to a crawl between Christmas and New Years. I couldn't care less about the holidays. I am curious if the slow down is entirely cultural, or if there is some kind of inherent coupling where we all naturally slow down with the longest winter nights, in places with significantly shorter daylight hours.

I've worked night shifts doing hard manual labor. I'm well aware humans can adapt to any rhythm when required. I'm curious about the effects on people that do not have such rigid lifestyles.