As I understand it, it's more of an internal clock that can be shifted by sunrise/sunset light/dark - but there appear to be a biological/inherited component. H

As I understand it, it's more of an internal clock that can be shifted by sunrise/sunset light/dark - but there appear to be a biological/inherited component. Hence some mammals have evolved a rhythm more dependent on seasons than day/night, due to polar day/night cycles: https://www.researchgate.net/publication/41942188_A_Circadian_Clock_Is_Not_Required_in_an_Arctic_Mammal https://www.researchgate.net/publication/41942188_A_Circadia... Which in turn might imply that humans still being rather young species is tuned to a rather strict day/night cycle, as seen close to the equator.