Abstract

Circadian rhythms are highly conserved across the tree of life, with light serving as the primary cue for the circadian timer. Photoperiod variation follows a latitudinal gradient, reflecting fluctuations in seasonality that increases towards the poles. As photoperiod variability intensifies at higher latitudes, organisms must develop mechanisms to maintain circadian timing during prolonged periods of light or darkness. Consequently, latitudinal gradients in photic sensitivity occur, as populations in northern latitudes have physiologically adapted to significant seasonal variations in day length. However, the rise of urbanization has introduced artificial light on a global scale, creating new photoperiods for organisms to navigate. This artificial extension of the natural photoperiod is now interacting with evolutionarily established circadian rhythms. Because photic sensitivity follows a latitudinal gradient, this may also inform biogeographic patterns of vulnerability to light pollution. We introduce the term "circadian histories" to describe an organism's evolutionary history of circadian entrainment, which may influence its response to novel light pollution.