You may be wondering why this chapter is titled "Daily and Circadian Rhythms." Daily rhythms are the same thing as circadian rhythms, aren't they? Yes and no. According to the American Heritage Dictionary, the adjective daily applies to something that happens or that is performed every day or once a day.1 A similar description is given for the adjective circadian (Figure 5.1). To understand the difference between the two words, one needs to look at their histories.
Daily derives from the Old English dœglic and dates back to the 12 th century.1 Halberg created the term circadian in the 1950s by combining the Latin terms circa (about) and dies (day).2 Thus, circadian literally means "approximately daily." During the 1950s and most of the 1960s, biologists argued about the existence of truly endogenous biological rhythms in the circadian range.3 Halberg — whose historical significance was discussed in Chapter 1 — was certain that circadian rhythms were endogenously generated, and he felt the need for a term that could emphasize the endogenous nature of the rhythms. He believed that the designation "approximately daily" (circadian) would convey the idea of endogenesis, because a process that can have a period different from that of environmental cycles must be produced endoge-nously. In conversations with me, Halberg recollected that
he created the term circadian in 1950 or 1951, although he did not formally introduce it to the scientific community until 1959.2 The new word became popular instantly. The U.S. National Library of Medicine's PubMed database lists 16 journal articles published in 1960 that include circadian in the title. The entries for over 1500 articles published by the end of 1969 can be retrieved by the term circadian in the title, abstract, or indexing field. The count surpasses 38,000 for entries through the end of 1999.
Halberg contributed to the confusion in using the new word by suggesting that circadian could apply to free-running rhythms and rhythms observed under a 24-hour environmental cycle.2 Of course, an endogenous rhythm does not stop being endogenous when it is synchronized to an environmental cycle, but a rhythm observed under a 24-hour environmental cycle need not be an endogenous rhythm. This subtle distinction eluded many researchers who were not specialists in circadian physiology. It was Aschoff — the second member of the triad of forefathers of circadian physiology discussed in Chapter 1 — who formalized three requirements for the appropriate use of the term circadian.4 A biological rhythm is said to be circadian if: 1) it is endogenously generated, 2) it has a free-running period close to 24 hours, and 3) it can be modified (synchronized) by environmental cycles with 24hour periods. Strictly speaking, all three requirements must be met to justify the use of the term circadian. (As mentioned in Chapter 4, a period "close to 24 hours" means a period between approximately 19 and 28 hours.)
The term daily — used to denote rhythms with a period of 24 hours whose endogenous nature has not been ascertained or has been disproved by experimental research — is not as unique as the term circadian. Many synonyms are currently in use. Daily is preferable to diurnal, which has been used in this sense by many authors512 but should be reserved for the meaning of "during the daylight segment of a day" (i.e., "during thephotophase"). Daily is also preferable to the unnecessary neologism diel, which has been used for many years, mostly by researchers with an ecological background.13-20 Another alternative is the term nycthemeral (from the Greek nychthemeron, which means "the duration of a night and a day"). Nycthemeral (or its French equivalent) has been in use since at least 188421 and has been adopted by many authors, particularly in Europe.22-28 Although I personally prefer daily to nycthemeral, the latter is probably a better technical term — because it does not have the double
meaning of "every day" and "once a day" that daily has, and because it follows the traditional derivation of scientific terms from the Greek or Latin. A much less elegant alternative is the use of the adjective 24-hour, such as in "24-hour rhythm."29-34
This section examines environmental rhythms with 24-hour periods, as well as some human populational rhythms (i.e., rhythms that can be detected at the level of groups of people but not in single individuals). Daily and circadian rhythms in individual behavioral (voluntary) functions are examined in Section 5.2, and rhythms in autonomic (involuntary) functions are examined in Section 5.3.
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