circadian

Etymology

It was coined by the German-born biologist Franz Halberg in 1959,^[2][3] formed from English circa + -an with Latin diēs (“day”) in between: from circā (“about, around”) and diēs.^[3][4][5][6] The Oxford Dictionary of English calls the formation irregular.^[7] Etymonline takes the middle part from the accusative singular of diēs,^[2] which goes back to Proto-Indo-European *dyeu- (“to shine”);^[2][6] circā is a form of circum (“round about”).^[2] Diēs is also behind diurnal.^[3]

Earliest recorded meaning

Of physiological activity recurring about every twenty-four hours: 1959, in Halberg's paper in the Zeitschrift für Vitamin-, Hormon- und Fermentforschung.^[3][4][5] Dictionary.com dates the word to 1955–60.^[8]

Semantic evolution

The first use was in the phrase circadian rhythm,^[2] and the adjective has kept to the biology of roughly daily cycles, which continue even when the surroundings give no clue to the time of day.^[9][3]

Timeline

• 1959 – of physiological activity, recurring about every twenty-four hours.^[3][5][4]

Meanings

Adjective

1. Of biological processes: occurring in cycles of about 24 hours, even without changes of light or temperature in the surroundings: Jet lag upsets the body's circadian rhythms.^{[9][5][3][10]}

Word family

• circadianly
• circadian rhythm

Related words

• circannual
• circa
• diurnal

References

1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
3. ^ a b c d e f g Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
4. ^ a b c Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
5. ^ a b c d Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
6. ^ a b The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
7. ^ Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
8. ^ Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
9. ^ a b Collins English Dictionary (2023), Glasgow: HarperCollins
10. ^ Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press