circa + dies

Circadian was coined in 1959 by the Romanian-born biologist Franz Halberg at the University of Minnesota. He combined Latin circa (about, approximately) and dies (day) to describe biological rhythms that cycle with a period of approximately 24 hours. The 'approximately' is the point. The human body's internal clock runs on a cycle of about 24 hours and 11 minutes. Without external cues like sunlight, the rhythm drifts. It is circa-dian — about a day, not exactly a day.

The discovery of circadian rhythms required experiments in isolation. In 1962, French geologist Michel Siffre spent two months in a cave without clocks, sunlight, or any time cues. His sleep-wake cycle settled into a rhythm slightly longer than 24 hours. Subsequent isolation experiments confirmed this: the human body has an internal clock that runs close to, but not precisely on, the 24-hour day. Sunlight resets it daily. Without sunlight, it drifts.

The suprachiasmatic nucleus (SCN) in the hypothalamus is the body's master circadian clock. It contains about 20,000 neurons that fire in a roughly 24-hour cycle, synchronized by light signals from the retina. Every cell in the body has its own circadian clock genes, but the SCN coordinates them. The 2017 Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash, and Michael Young for discovering the molecular mechanisms of circadian rhythms in fruit flies.

Jet lag, shift work disorder, and seasonal affective disorder are all circadian rhythm disruptions. The modern world — with its artificial light, time zones, and 24-hour economies — constantly fights the circadian clock. The body wants to follow the sun. The economy wants to ignore it. The Latin 'about a day' describes not just the clock but the struggle.