bio

Circadian rhythm

A biological rhythm that oscillates on its own with a period of roughly - but almost never exactly - one day. The name comes from the Latin *circa diem*, "about a day," and that "about" is the whole point. The rhythm is not a response to light: it is generated internally and keeps running in total darkness, as the astronomer de Mairan discovered in 1729 when he shut a sensitive plant in a cupboard and its leaves went on opening every morning. A genuine circadian rhythm must satisfy three conditions: it oscillates without external cues, it can be reset by environmental signals, and its period holds steady as temperature changes.