A circadian rhythm is a roughly-24-hour cycle in the physiological processes of plants, animals, fungi and cyanobacteria. (The term "circadian" comes from the Latin circa, "around", and dies, "day", meaning literally "around a day.") It was initially discovered in the movement of plant leaves in the 1700s by the French scientist Jean-Jacques d'Ortous de Mairan. The formal study of biological temporal rhythms such as daily, weekly, seasonal, annual, is called chronobiology.
The circadian rhythm partly depends on external cues such as sunlight and temperature. Early researchers observed that some sort of "internal" rhythm must exist, because plants and animals did not react immediately to artificially-induced changes in daily rhythms. However it has been well established that a mechanism for adjustment also exists, as plants and animals eventually adjust their internal clock to a new pattern (if it is sufficiently regular and not too far off the norm for the species). Overall, circadian rhythms are defined by three criteria:
The rhythm persists in constant conditions (for example constant light) with a period of about 24 hours
The rhythm period can be reset by exposure to a light or dark pulse
The rhythm is temperature compensated, meaning that it proceeds at the same rate within a range of temperatures.
man... my circadian rhythm is fucked up!