Press release

The Circadian Clock Revisited: Peripheral Clocks are Light-Sensitive

Paris, March 1, 2000


CNRS – INSERM Joint Press Release

The circadian clock is found in all living creatures. It regulates our physiological systems and when not functioning correctly can be the cause of illnesses and syndromes such as sleep disorders in humans. It has long been claimed that there is a central, or even unique, clock that is not influenced by light. But research by Paolo Sassone-Corsi and his team (CNRS-INSERM-ULP* ) has turned this theory around. In 1999, they showed the existence in vertebrates of peripheral clocks in a variety of tissues and independent cells. Today, their research shows that these peripheral clocks seem to be directly controlled by light. Researchers are now searching for the photoreceptors responsible for activating the biological clock. The results of this research are published in Nature** (2 March 2000).

Initial research into the molecular clock in animals concentrated on Drosophila, already suggesting that certain tissues were able to act independently of the central clock located in the nervous system (the hypothalamus); this idea went against commonly accepted knowledge. Prior to this, the central clock was thought to be responsible for the circadian rhythms and other rhythmic phenomena in the organism. It has now been shown that independent clocks may exist in other regions of the brain.

In 1999, the results of research on zebrafish conducted by Paolo Sassone-Corsi and his team showed that vertebrates possess peripheral clocks. The expression of a gene similar to the Clock gene in mice***, which is involved in the functioning of the biological clock, oscillates according to a circadian rhythm in almost every tissue of the fish, whether the tissues are observed in vivo or in culture. This provided proof of the existence of peripheral clocks.

Today, researchers have shown that these peripheral clocks are sensitive to light. An experiment performed on kidney and heart cells in culture using a day/night rhythm revealed a more robust oscillation of the expression of the gene. When the rhythmicity was reversed, the oscillation was also reversed. The structure of the peripheral clocks can thus be described as photosensitive. Light acts directly on the circadian oscillator in the peripheral tissues. These observations led to the concept of a clock at the cell level and suggested the presence of photoreceptors at the surface of the heart and kidney cells. These photoreceptors are part of the circadian clock.

This was an unexpected result. It was thought that in vertebrates the circadian clock depended on circadian photoreceptors in the eye or on the pineal gland. The circadian system in vertebrates does then exist in a decentralized way. There are still some questions that need answering, however: Do the independent peripheral clocks exist in mammals? What role do they play? Do they influence the organism or are they simply oscillators that are coordinated and controlled by the central clock?

* Université Louis Pasteur, Strasbourg
** Whitmore D., Foulkes N. S. and P. Sassone-Corsi. Light acts directly on organs and cells in culture to set the vertebrate circadian clock. Nature, 2 March 2000
*** the only gene involved in the vertebrate biological clock

Researcher Contact:
Paolo Sassone-Corsi
Institut de Génétique et de Biologie Moléculaire et Cellulaire
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Contact at the CNRS Life Sciences Department
Thierry Pilorge
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Press Contact:
CNR - Stéphanie Bia
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