Jean-Jacques d'Ortous de Mairan (November 26, 1678 – February 20, 1771), a French geophysicist, astronomer and most notably, chronobiologist, was born in the town of Béziers on November 26, 1678. De Mairan lost his father, Francois d'Ortous, at age four and his mother twelve years later at age sixteen.

Over the course of his life, de Mairan was elected into numerous scientific societies and made key discoveries in a variety of fields including ancient texts and astronomy. His observations and experiments also inspired the beginning of what is now known as the study of biological circadian rhythms. At the age of 92, de Mairan died of pneumonia in Paris on February 20, 1771.

De Mairan attended college in Toulouse from 1694 - 1697 with a focus in ancient Greek. In 1698 he went to Paris to study mathematics and physics under the teachings of Malebranche. In 1702, he returned home to Béziers and began his life long study of several fields, most notably astronomy and plant rhythms. Furthermore, during his time in Béziers, he ate every day with Cardinal de Fleury; later, de Mairan founded his society under the protection of Cardinal de Fleury. Eventually, de Mairan received official lodging in the Louvre where he remained pensionnaire until 1743 and served as secretary from 1741 to 1743. In 1746, he was reinstated as pensionnaire geometre, or full time boarding surveyor. It is reported that the Prince of Conti and other great lords heaped extravagant gifts upon him. He was also secretary to the Duke of Orleans.

• In 1719, De Mairan discussed the varying obliquity of light that causes cold in winter and heat in summer. He postulated that the sun’s heating effect was related to the square of the sine of its elevation. He neglected the effects of atmosphere, admitting that he did not know how much the sun’s heat would be absorbed by it. Two and a half years later, he presented a paper to the Academie Royale des Sciences in Paris: “Problem: the ratio of two degrees or quantities of sunlight seen through the atmosphere at two different known angular elevations being given, to find what part of the absolute light of the sun is intercepted by the atmosphere at any desired elevation.” In this paper, de Mairan made a hypothesis based on mere observations, supposing that the ratio had been measured, even though it had not. The significance of de Mairan’s work, although incorrect, led his protégé, Pierre Bouguer, to invent the photometer.
• In 1729, de Mairan constructed an experiment showing the existence of a circadian rhythms in plants, presumably originating from an endogenous clock, or the rhythms internally generated by an organism.
• In 1731, he also observed a nebulosity around a star near the Orion nebula. This was later designated M43 by Charles Messier.

In 1729, de Mairan performed an experiment that demonstrated the existence of circadian rhythms in plants, specifically the Mimosa pudica. He was intrigued by the daily opening and closing of the heliotrope plant and performed a simple experiment where he exposed the plants to constant darkness and recorded the behavior. De Mairan's key conclusion was that the daily rhythmic opening and closing of the leaves persisted even in the absence of sunlight. However, de Mairan hesitated to conclude that heliotropes have internal clocks and hypothesized that other factors, such as temperature and magnetic fields, were responsible for the rhythmic behavior. He did not publish his results because he doubted his findings and the importance of their implications.

These results may have gone unnoticed had his colleague, Marchant, not published them for de Mairan. The published accounts of de Mairan's work stimulated further research in the field of chronobiology.

Despite Marchant's publication of de Mairan's work, which was clear evidence for intrinsic circadian oscillations, rhythms in plant movements were thought to be extrinsically controlled, by light and dark cycles, or magnetic and temperature oscillations, for more than thirty years. As more researchers confirmed and replicated de Mairan's result that plant rhythms persisted in constant darkness it became clear that the rhythms continued even while controlling for other possible influences, such as temperature.

In 1823, over a century later, the Swiss botanist Augustin Pyramus de Candolle expanded on de Mairan's early theory on the circadian nature of M. pudica by measuring the free running period of plants in constant darkness, finding them to be 22 – 23 hours long. These results, when considered with the growing experimental evidence of circadian rhythms in many organisms, reinforced the idea that a circadian rhythm could be found in all living organisms, a belief widely held today among chronobiologists.

The circadian nature first described by de Mairan in plants is now evident in a variety of animal models from Drosophila to the common laboratory mouse to humans. Even when describing his work with rhythmic eclosion times in his fly models or running activity of mice, founder of modern chronobiology, Colin Pittendrigh, recognized the work of Jean - Jacques d'Ortous de Mairan. The circadian rhythmicity, which de Mairan first described in Mimosa leaf movements, is now recognized as a feature nearly ubiquitous across all phyla.

In 2011, researchers at the University of Tsukuba in Japan examined an ortholog of Gigantea (GI), a key regulator of the flowering time in plants, to reveal key features of the flowering in plants. They isolated the GI ortholog PnGI from a short period (free running period less than 24 hr.) plant, Pharbitis (Ipomoea) nil. The mRNA expression of this ortholog showed diurnal rhythms that peaked at dusk under short and long day conditions; it also showed circadian rhythms under continuous light and continuous dark conditions. Their data also suggested that PnGI functioned as a suppressor of flowering through its down - regulation of PnFT1, a gene that induces the flower to bud given a single dusk signal.

In 1718, de Mairan was inducted into the Académie Royale des Sciences. The Cardinal and the Count of Maurepas selected Mairan to replace Fontenelle as Associate Secretary of the Academie in 1743. De Mairan also served as the Academie's assistant director and later director intermittently between 1721 and 1760. Eventually, de Mairan was appointed editor of the Journal des sçavans, a science periodical, by Chancellor d'Aguesseau. Also, in 1735, de Mairan was elected a Fellow of the Royal Society and in 1769, a Foreign Member of the Royal Swedish Academy of Sciences as well as to the Russian Academy (St. Petersburg) in 1718. De Mairan was also a member of the Royal Societies of London, Edinburgh, and Uppsala and the Institute of Bologna. With Jean Bouillet and Antoine Portalon, he founded his own scientific society in his hometown of Béziers around 1723.

Beyond astronomical and circadian observations, de Mairan actively worked in several other fields of physics including "heat, light, sound, motion, the shape of the Earth, and the aurora".