The moment when Earth’s Northern Hemisphere tilts farthest away from the sun—the December solstice, on the 21st this year—is not just a mark on the calendar. It is also defined by the way our bodies react to the event. The dimming of our daily ration of natural light leading up to the winter solstice produces a series of conspicuous physiological changes.
These changes relate to circadian rhythms. The word circadian derives from the Latin “circa diem,” meaning “about a day.” It signifies the way animals, plants, fungi and bacteria react to environmental cues, including inputs of light, on a daily and seasonal basis.