THE BIOLOGY BOOK

Circadian Rhythms

1729

In 1729, the French scientist Jean-Jacques d’Ortous de Mairan noted that mimosa plants folded and unfolded their leaves following a regular schedule, over a 24-hour period, even when kept in total darkness. This was the first scientific description of circadian rhythms (CR). These daily light-dark cycles appear at approximate 24-hour intervals and are named circadian (“around a day”). CR govern rhythmic biological changes and are self-sustaining even in the absence of environmental cues. Synchronization of an organism to both the external and internal environment is vital to that organism’s well-being and, indeed, its survival.

CR are not unique to plants. It was studied in the 1950s in Drosophila (fruit flies) by Colin Pittendrigh and in humans by Jürgen Aschoff and was also observed in fungi, animals, and cyanobacteria (blue-green algae). CR are important for the sleep-wake cycle and the feeding patterns of all animals and are accompanied by more subtle patterns involving changes in gene activity, brain wave activity, hormone production and release, and cell regeneration. Disruption of CR has adverse effects on health, as is observed in those with “jet lag” who experience fatigue, disorientation, and insomnia.

In mammals, the circadian master clock is found in the suprachiasmatic nucleus (SCN), which is located in the hypothalamus. Information about incoming light is processed by the SCN and transmitted from the retina in the eye to the pineal gland. The pineal controls the production and secretion of the hormone melatonin, which peaks at night and ebbs during the daytime. Working in concert with the biological clock and serving as the molecular basis for CR is an internal clock, which consists of genes that oscillate in their activity and, hence, their protein products, and these regulate biological functions in the body.

In a manner similar to that seen in animals, in the absence of environmental signals, such as changes in light, temperature, or humidity, plants exhibit circadian fluctuations. These have been seen in their photosynthetic activity as well as leaf movements, opening of flowers, germination, growth, and enzyme activity. Once again, the underlying basis for CR appears to revolve around the activity of genes.

SEE ALSO: Regeneration (1744), Phototropism (1880), Secretin: The First Hormone (1902), Thyroid Gland and Metamorphosis (1912), Insulin (1921), REM Sleep (1953), Hypothalamic-Pituitary Axis (1968).

The first recorded observation of the circadian clock occurred in 1729 when a French scientist noticed how mimosa plants opened and folded their leaves over 24-hour cycles.