It is said that when Louis "Satchmo" Armstrong was asked, "What is rhythm?" he replied, "If you ask, you ain't got it!" This pithy squelch cannot be extrapolated to anyone posing the same question concerning circadian (about-24-hours) rhythms. In common with most, possibly all, living organisms, we possess biological rhythms susceptible to environmental cyclic changes, yet basically endogenous.
Periodicity of some biological processes has always been apparent, but its general acceptance as a fundamental "law of nature" did not encourage inquiry. Curiosity about circadian rhythms is recent, largely prompted by their observed desynchronization after time-zone changes in the course of flight. Passengers and crew of such flights experience discomfort, fatigue, and loss of efficiency in adjustment to a new timetable. They also manifest derangement in circadian excretion patterns of 17-hydroxycorticosteroids and electrolytes. Adaptation may take several days. Similar rhythm dislocations occur in submarine crews, astronauts, and— less dramatically—in industrial workers
ABOUT-24-HOURS RHYTHMS. JAMA. 1968;204(3):258-259. doi:10.1001/jama.1968.03140160068021