The study of circadian rhythms dates back to the 19th century. However, significant studies and observations began in earnest in the 1960’s. One early pioneer was Curt Paul Richter, a professor of psychobiology at Johns Hopkins Medical School whose innovative concepts are the foundation for established concepts and methods for studying circadian rhythms in humans.

The word circadian is Latin in origin from the word circa meaning “about” or “approximately” and dian meaning “day”. The term circadian was coined when Franz Halberg, a scientist at the University of Minnesota, published a paper in 1959 showing blood count varied according to a strict rhythm that was about a day. In fact, Halberg discovered that the rhythms actually went somewhat longer than a twenty-four hour day.

The field of chronobiology, the medical discipline that studies circadian rhythms, has since revealed that the body undergoes more than just blood count variations. In fact, the body makes significant changes throughout a day.