For most of life on Earth, sunrise and sunset coincide with daily activity patterns. However, biological organisms have internal clocks that orchestrate their daily rhythms even in the absence of the sun. These circadian rhythms, which define the timing of the sleep-wake cycle, are the reason people experience jet lag when crossing time zones. At the cellular level, circadian rhythms control patterns of metabolism and other biological processes that fluctuate over the course of a day.
The existence of daily rhythms has been observed since ancient times, but the first evidence that these patterns are defined by an internal mechanism, rather than by sunlight alone, came from French scientist Jean-Jacques d’Ortous de Mairan in 1729 (1). After observing that mimosa plants open their leaves during the day and close them at night, Ortous de Mairan wondered how the plants would behave in total darkness. He found that mimosa plants kept in total darkness continued to open and close their leaves at more or less the same rate.
In a short time, other researchers observed internal daily cycles in plants and animals, including humans, but the biological processes underlying circadian rhythms would remain a mystery for the next two centuries. Current understanding of circadian rhythms, as well as the field of circadian biology, owes its beginnings to a now classic 1971 PNAS article, “Clock mutants of Drosophila …