User:Eugene M. Izhikevich/Proposed/Diurnal

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Figure 1: Definitions of physiological temporal characteristics. Note that diurnal is here restricted to the illuminated part of the day; also circadian can differ in length, being short in E. coli (1) and in an archaeon (2) (shaded horizontal arrow with two heads indicating that limits are only an approximation). 1. Halberg F, Conner RL. Circadian organization and microbiology: Variance spectra and a periodogram on behavior of Escherichia coli growing in fluid culture. Proc minn Acad Sci 1961; 29, 227-239. 2. Halberg F, Cornélissen G, Sothern RB, Barnwell F, Cegielski N, Ilyia E, Siegelova J. The moon's and the genes' tides and double tides pulling the biosphere. In: Halberg F, Kenner T, Fiser B, Siegelova J, eds. Noninvasive Methods in Cardiology, September 16-17, 2010, Brno, Czech Republic. Brno: Faculty of Medicine, Masaryk University. p. 23-45. © Halberg.

Diurnal (adj.; diurnally adv.; diurnality noun; from Latin diurnalis, referring to day: di from dies + urnalis from Latin adjective-forming suffix): relating to variations or events, biological and other, occurring between sunrise and sunset or during the illuminated fraction (photofraction) of a near-daily schedule of solar or artificially induced alternation of light and darkness. The added traditional transdisciplinary use of diurnal to refer to the time between two sunrises is discouraged. As a noun, diurnal is on occasion used for a diary, journal, newspaper, and in a chronobiological context for symptomology, activity or a drug recommended for use or occurring by daytime only, as in diurnality and diurnal (vs. nocturnal) epilepsy, asthma or filariasis.



The term diurnal has traditionally been used with two distinctly different meanings: for the time between sunrise and sunset, and for the time between two sunrises.
Figure 2: This figure shows the recommended uses of the terms diurnal vs. circadian. The acrophases shown can be described as circadian, with four of them (top) diurnal and the one at the bottom nocturnal. Hence, diurnal should not be used to describe a full daily cycle. Circadian is preferred to nycthemeral, still another term, now somewhat archaic, also used to describe a 24-hour or other alternation of night and day verbatim or of an about 24-hour span, yet nycthemeral conveys neither the statistical nor the partly built-in nature of the about 24-hour rhythms (Halberg, 1969; Halberg et al. 1977), in keeping with decisions of international nomenclature committees. In this graph, the top four acrophases are diurnal and the one at the bottom is nocturnal. © Halberg.
A restriction in the meaning of diurnal to the first sense was suggested by international nomenclature committees for several reasons. First, different terms are desirable for clearly different time spans, such as those of, e.g., about (~) 24 hours vs. of ~12 hours. Restricting diurnal to the illuminated fraction of the day emphasizes the difference between the illuminated and the darkened (photo- and scoto-) fractions of a 24-hour or other span, involving an alternation of light and darkness in nature or in the laboratory, Figures 1 and 2. By adjectives such as diurnal and nocturnal and by nouns like "diurnality" and "nocturnality", it becomes possible to characterize briefly certain general features of bioperiodicity in phenomena dealt with in physics as well as in general biology, in zoology, as occurrence or activity during the daytime rather than at night or vice versa; in botany, as opening of flowers during the photofraction and closing during the scotofraction; and in biomedicine, as characterizing the time patterns, e.g., of convulsive disorders or of body activity, occurring mainly by day-time, as opposed to events occurring mainly during other parts of the 24-hour span. The same distinction between nocturnal and diurnal is also made, among many other phenomena between the (nocturnal) microfilarial periodicity of Wuchereria bancrofti vs. that of (diurnal) Loa loa, with high filarial counts in blood by night or by day, respectively.

History of the term

Franz Halberg had originally proposed that the use in biology of "circadian", which he had just coined in 1950, be restricted to rhythms desynchronized from precisely 24 hours or from precisely 1 year, free-running from their schedule with which they are usually locked into sync. Hence he had also suggested the additional term dian for 24-hour synchronized rhythms. Committees of the International Society for the Study of Biological Rhythms (later the International Society for Chronobiology) and a committee of the International Union of Physiological Sciences voted for a single term for both kinds of rhythm, namely circadian, and advocated the use of diurnal for daylight hours (Figures 1-8), so as to avoid the double use of diurnal for both "24 hours" and for "part of the day", in a phrase like "a diurnal [24-hour] rhythm in diurnal [mostly daytime] epilepsy", where the same adjective stands for two different time spans; an elaboration, e.g., in figure headings by the words in the foregoing parentheses, would be cumbersome.

Figure 3: Diurnal epilepsy: unequal distribution of convulsive seizures during the hours of the day and night. Note statistical nature of clinical diurnality in human convulsive disorder, here displayed time-macroscopically for an individual patient. For complementary time-microscopy see Figure 3-Figure 3. © Halberg.
Figure 4: Circadian group rhythm of seizures in patients with convulsive disorder plotted time-macroscopically (left) and analyzed time-microscopically (by population-mean cosinor, right). An elliptical 95% confidence region that does not cover the pole (center) documents the rejection of the zero-amplitude assumption (in keeping with a rhythm with a period near 24 hours used for testing). Tangents drawn to the error ellipse provide for the uncertainty of phase; intersects of the ellipse with the length of the (amplitude) vector provide an uncertainty estimate for amplitudes tabulated on the right. © Halberg.

The examples of Figures 2-8 are all of circadian rhythms, not of diurnal ones, although many scholars unfamiliar with the history of the field or with medical practices (at this time not the majority) insist on the use of "diurnal" or "nycthemeral" to describe a 24-hour synchronized biological rhythm and restrict the use of "circadian" to desynchronized rhythms (as originally suggested by Halberg but rejected by committees on nomenclature). If the practice of the designation of a rhythm as synchronized or desynchronized from a given, e.g., 24-hour routine were generally followed, e.g., in health care, it would be cumbersome as yet insofar as it would require prior long-term monitoring in special environments and, in that case, perhaps more than one term, e.g., dian vs. circadian could be reconsidered (or diel vs. dieloid, also proposed by Halberg in 1955 but rejected by an international nomenclature committee). New technology for automatic monitoring of physiological functions and software for as-one-goes analysis may render such a dichotomy implementable, albeit not soon in general health care practice. At this time, in the majority of biomedical articles, circadian is used for both the synchronized and the desynchronized cases in biology and medicine and diurnal is mostly restricted to daytime, but the need to arrive at a consensus in all of science, including in particular physics, remains, unless one follows the practice of using a definition of terms in each report and allows these definitions to vary from one report to the next. The community of physicists has traditionally used diurnal to mean, "performed in or occupying one day; daily", notably in an astronomical context. But even in reference to physical matters such as environmental temperature, it seems awkward to say "a diurnal temperature rise during the day and diurnal fall at night". Usage of circadian or at least dian may also be more appropriate in this case and may also account for day-to-day variability.

Figure 5: Individualized single cosinor assessment of seizure incidence in two patients on the same hospital routine. Such information on both phase (angle) and amplitude (length) of the directed line, respectively, with their uncertainties, may serve for the timing and dosing of medication and for study of etiology. © Halberg.
Figure 6: Display only of phase from individualized single cosinor assessment, as shown in Figure 5. Point estimates of phase are given with their 95% confidence arcs, except for cases when the hypothesis of no circadian rhythm cannot be rejected. Then, only a point estimate is provided as a circle. © Halberg.

Whereas it may be argued that the correct meaning of diurnal may be inferred by the context in which the term is used, ambiguity is more likely to occur with a term of multiple meanings such as diurnal that could easily be interpreted correctly or incorrectly even within a given context. In view of the critical requirement of exactitude in the scientific literature, repetitions being preferred to elegant variations whenever two terms do not have a fully equivalent meaning, reserving diurnal to relate to the lighted part of the day and using circadian to mean the about-24-hour frequency of events thus seems to be preferred to avoid any ambiguity while remaining succinct. In the examples of Figures 2-8, considering the broader meaning of the term, reference to the diurnality of epilepsy could equally be interpreted as a condition with an about-24-hour pattern (that could involve seizures by night) or as events occurring only during the illuminated part of the day, opposite implications by the use of the same term. Indeed, some patients with convulsive disorders have seizures restricted to daylight hours, others have convulsions only at other times, and yet others do not show a relation to any special part of the 24-hour scale. Hurdles remain with those who now echo earlier suggestions by restricting "diurnal" to 24-hour synchronized rhythms in biology and with a long-standing use in this context of "diurnal" in physics.

Figure 7: Time-macroscopic (left) and time-microscopic (right) display of individualized single cosinor assessment of circadian rhythm in seizure incidence of six patients. Note that in one case the single cosine fit does not reject the zero-amplitude (no-rhythm) assumption. A more complex model than a single 24-hour cosine fit is particularly indicated in such a case, but may serve in all cases. © Halberg.
Figure 8: The percentage of total recorded time of paroxysmal discharge determined on hard copy EEGs in the 1950s undergoes a circadian rhythm, as was predicted for a patient (JS) with an acrophase at -348° (23:12) and a 95% confidence arc extending from -275° to -60° (18:20 to 04:00), thus including the point estimate of -26° (01:44) for JS in Figure 7. The excessively wide arc is a reason for caution in curve fitting to relatively short, in this case 24-hour time series and awaits the development of technology for long-term, at least week-long EEGs, as well as in considerations of time lags between the average times of onset of EEG-revealed pathology and of seizure incidence, although the 95% confidence intervals for the two variables greatly overlap one another for JS. Not only seizure incidence, but the temporal placement of circadian rhythms in pathology shows circadians and the acrophase differences between EEG and overt pathology may provide clues about mechanisms and may be considered in timing medication. © Halberg.


  1. Halberg, F. (1969). Chronobiology. Annu Rev Physiol 31: 675-725.
  2. Halberg, F., Cornélissen, G., Katinas, G., Syutkina, E., Sothern, R., Zaslavskaya, R., Halberg, F., Watanabe, Y., Schwartzkopff, O., Otsuka, K., Tarquini, R., Frederico, P., and Siggelova, J. (2003). Transdisciplinary unifying implications of circadian findings in the 1950s. J Circadian Rhythms 1(2). Page 61. online
  3. Halberg, F., Cornélissen, G., Bingham, C., Witte, H., Ribary, U., Hesse, W., Petsche, H., Engebretson, M., Geissler, H., Weiss, S., Klimesch, W., Rappelsberger, P., Katinas, G., & Schwartzkopff, O. (2003). Chronomics: Imaging in time by phase synchronization reveals wide spectral-biospheric resonances beyond short rhythms. Neuroendocrinol Lett 24: 355-380.

Further reading

See also

Circadian rhythms, Chronobiology, Chronomics

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