Day and night effects on the animal and plant kingdoms: The eve of chronobiology

ABSTRACT

For a long time, cyclical changes in the body were assumed to be caused by the cyclicity of the environment (day-night, seasons). The concept of daily and seasonal changes was first documented in the 18th century by astronomer D’Ortous de Mairan, who demonstrated that plant leaf motions varied depending on the time of day, and by Linné’s description of his floral clock in 1751. In 1832, De Candolle was the first to experimentally establish the endogeneity of rhythms in plants, underlining the notion of what we now term free-running rhythms. Julien-Joseph Virey made his own contribution in his thesis, published in 1814, against this backdrop, in which he examined the knowledge of his day on the daily and seasonal biological fluctuations of living matters. He emphasized the relevance of the environment’s day-night cycle on plant life and created a list of plants based on their diurnality or nocturnality. He expanded on the issue of rhythmic changes in human health and sickness and provided his own data on the daily fluctuations in patient mortality he discovered at the Val-de-Grâce military hospital where he was chief pharmacist. What is crucial is his use of terms such as “living clock,” “entrainment,” and “innate rhythm” and the applicability of the advanced concepts Because Virey introduced the notion of temporal variations and the impact of the alternation of day and night on these variations, this thesis is a historic testimonial to understanding of biological rhythms in the first half of the 19th century. We may assume from his writings on rhythmic fluctuations that he offered the theory, followed by an experiment, however primitive, from which he drew conclusions and postulated a mechanism (the living clock) that would later prove accurate. All of these aspects indicate that this study represents an early exploration of the notion of temporal variations in humans.

KEYWORDS:

• History of chronobiology
• nychthemeron
• internal clock
• environment
• light
• darkness
• biological rhythms
• Virey
• de Mairan
• de Candolle

Acknowledgments

The support of the Thérèse Pontremoli donation for chronobiology research at the Fondation A. de Rothschild (Paris) is hereby acknowledged. I would like to thank Pr Bernard Millet (Université de Besançon, France) for useful discussion and Jerôme van Wijland, Director of the library of the Académie de médecine (Paris), for showing me the academic file of JJ. Virey, fellow of this Academy.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

The work was supported by the Fondation A. de Rothschild .