Chronobiology is the study of biological rhythms with respect to time. The most important rhythm studied in chronobiology is the circadian rhythm, which occurs over approximately 24 hours. Circadian rhythms regulate many physiological processes and can be broken down into daily cycles like diurnal (day active), nocturnal (night active), and crepuscular (dawn and dusk active). The history of chronobiology dates back to the 18th century when de Mairan described circadian rhythms in plants, and Franz Halberg coined the term "circadian" in the 20th century. Biological rhythms can also occur on longer (infradian) or shorter (ultradian) time scales than 24 hours.