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Time rhythms can be found throughout the living realm. Circadian (from Latin circa - around, dies - day) or approximately daily rhythms have been mentioned several times in the magazine Světlo - e.g. in issues 1 and 3/2005 or 5 and 6/2008. The term circadian was introduced in the 1950s by Franz Halberg, one of the founders of chronobiology, the science of time order in the living realm. Among Czech chronobiologists, the best known is Prof. Helena Illnerová, who with her colleagues, using a rat model, was the first in the world to discover how the change in the length of illumination during the changing seasons affects the rhythm in melatonin production in the pineal gland [1] and in photosensitivity in the biological clock in the suprachiasmatic nucleus of the hypothalamus [2].

In the mammalian central nervous system, the suprachiasmatic nuclei (SCN), the central biological clock that controls, among other things, blood hormone levels, body temperature, sleep and wakefulness, are located below the optic nerve junction. Melatonin is the hormone of sleep and bodily regeneration. Cortisol, on the other hand, is the hormone of activity, stress and movement. An example of a waveform of monitored levels, taken from [3], is shown in Figure 1. The waveforms for each day are variable in both axes. The central clock is adjusted each day by the effect of light, but food intake also has an effect. Without adjustment, this clock would run freely in a young person with a period of approximately 24 hours, hence the term circadian. According to this central clock, the internal clocks of the individual organs are further synchronized. The stimulus for the adjustment of the central clock can be light of a suitable spectral composition in the order of as little as a lux for a period of minutes, followed by a drop in the level of melatonin in the blood.

Effects of light on living organisms

These effects have been studied in detail by the German ophthalmologist Prof. Fritz Hollwich, author of a textbook on ophthalmology and many treatments. In his 1948 habilitation thesis he distinguished between the visual and the energetic (non-visual) components of the ocular apparatus. He found different levels of certain hormones and other substances in patients blinded by cataracts compared to levels common in the healthy population and observed the return of these levels to normal after surgical lens replacement, when the patients regained their vision. He also found that certain types of light, excess, deficiency or prolonged invariability, have adverse effects on animals [4]. In the last few years, there has been talk of a new photoreceptor, the light-sensitive retinal ganglion cell (ipRGC). In mice they were discovered in 1991, in humans only in 2007. They contain the dye melanopsin and their maximum sensitivity is reported in the wavelength range 450 to 482 nm (rarely also 420 and 491 nm), see Figure 2. These cells give the central clock a stimulus to adjust, are involved in the pupillary constriction reflex and possibly in shaping visual perception. They are found throughout the retina and are more abundant in the lower part of the retina. Collectively, they may be called the circadian sensor. Because of their sensitivity to blue light and their distribution on the retina, they are referred to as the blue sky detector. Recent research [5] shows that both the circadian sensor and cones are involved in the synchronization of the central clock and, in addition, exposure time plays a role. Comparing narrowband radiation of dominant wavelengths 460 and 555 nm, it has been shown that their effect on melatonin decline is initially about the same, but almost disappears within 90 minutes with green light, whereas with blue light the effect is permanent. C1(λ) rather describes the sensitivity under long-term exposure and C2(λ) partly accounts for short-term exposure. Two types of effect are reported: a decrease in melatonin levels and a phase shift of the central clock.

In the literature [6] and [7] the design of a circadian dosimeter (Daysimeter, LuxBlick) is discussed. It is a small device that can be worn similar to glasses. Two photodiodes are used as detectors - the sensitivity of the first is corrected to V(λ) and the second to C(λ). The measured illuminance values are stored in memory together with time stamps at intervals of tens of seconds. By analysing the measured values, it can be determined whether the user receives the necessary dose of light effective for the nervous system during the day and whether he is not disturbed by the light at night. Critical areas can be located over time and appropriate remedial measures can be suggested. The identification of circumstances can be facilitated by data from additional sensors, such as an accelerometer or thermometer. The lowering of melatonin in the morning and the maintenance of low melatonin levels during the day can be considered more than desirable as it triggers a number of processes leading to greater alertness, activity and concentration. The enhancement of the circadian sensitivity spectrum can be achieved by using light sources with higher chromaticity temperature. According to the Kruithof diagram, users can then be expected to demand higher illuminance, which can be addressed, for example, by additional local luminaires. Higher illuminance and higher chromaticity temperature can have a concrete economic output in workplaces in the form of better quality of work [8], reduced stress [4], better use of working time or reduced sickness.

Melatonin is the hormone of sleep and regeneration of the body. It "scavenges" free radicals in the body and destroys cancer cells. It is therefore extremely beneficial to let it work undisturbed at night. Several measures can be taken against disturbing light at night, from sophisticated outdoor lighting fixtures to blinds, curtains and shutters to red night lighting.

White LEDs are mostly blue LEDs with a phosphor, which converts the blue light into yellow and transmits it. Here also lies a certain risk of disturbing the darkness of the night with LED-based public lighting. Blue light scatters more in the atmosphere than radiation of longer wavelengths. Attention must therefore also be paid to the issue of scattered light interference.

According to publication [9], LEDs with a low chromaticity temperature (2 600 K) are the most suitable for public lighting in this respect, but even here the proportion of circadian effective radiation is three to four times higher than for commonly used high-pressure sodium lamps (see Table 1).

Calculation and measurement

In [10] circadian quantities are introduced in analogy to photometric quantities. The function V(λ) is replaced by C(λ) and the quantities are given the index c. Thus, it is possible to work with the concept of circadian illuminance, for example. Circadian illuminance can be measured with a luxmeter corrected for the relative circadian efficiency C(λ). For indicative measurements, the dark blue foil Lee No. 120 can be used for correction. Another possibility is to calculate from the measured radiant flux spectrum or to find the conversion factor for a given source. According to [10], a circadian efficiency factor acv (German: circadianer Wirkungsfaktor) can be introduced, which is calculated for light with a relative power spectral composition X(λ) according to relation (1):

acv is therefore the coefficient for a given light source for converting photopic values to circadian values and can be used to compare different lights, or light sources, in terms of their effects on the nervous system.

The course of the curve C(λ) and the content of the area under it are not yet known exactly. It is therefore appropriate to provide the calculation with a coefficient which will make the values calculated comparable with the current and future updated C(λ) curve. The coefficient can be defined in various ways, e.g. by the equality of the areas under C(λ) and under V(λ) or by the equality of the luminous and circadian flux for CIE standardised light A (incandescent light model). The variant I propose for discussion has the working title circadian activation index and the working designation Ac. Its value is set to 100 for CIE D65 light and is calculated according to relation (2):

Ac allows to compare light sources in terms of their circadian effect. For reference - daylight has a value of 100. Its values can easily be calculated for common types of light sources and blackbody temperatures (Table 1).

In addition to the daily rhythms, there are known physiological rhythms such as tidal rhythms and weekly, monthly and annual rhythms. Lack of light in winter contributes not only to winter sleepiness and the need for longer sleep, but also to seasonal affective disorder (SAD), also known as winter depression. Here, among other things, intensive light therapy (phototherapy) is used. Exposure to 10,000 lx illumination at eye level for 30 minutes has been shown to be effective [11]. So-called solar simulators are designed for personal use. In contrast to industrial sunlight simulators, these are luminaires for eye and face illumination. Known are table or wall lamps with fluorescent tubes (the usual illumination is 10 klx on the diffuser surface) or pocket battery-operated lamps with white or blue light-emitting diodes. Less well known are the so-called light visors, caps with built-in LEDs to illuminate the eyes. These aids can make it easier for the wearer to step into the day, but for a lasting effect, all-day lighting that is also suitable in terms of its circadian effect is essential.

Literature:

[1] ILLNEROVA, H. - VANECEK, J.: Pineal rhythm in N-acetyltransferase activity in rats under different artificial photoperiods and in natural daylight in the course of the year. Neuroendocrinology, 1980, 31, pp. 321-326.

[2] SUMOVA, A. - TRAVNICKOVA, Z. - PETERS, R. - SCHWARTZ, W. J. - ILLNEROVA, H.: The rat suprachiasmatic nucleus is a clock for all seasons. Proc. Natl. Acad. Sci., USA, 1995, 92, pp. 7754-7758.

[3] BOMMEL VAN, W. J. M. - BELD VAN DEN, G. J. - OOYEN VAN, M. H. F.: Industrial lighting and productivity. Philips Lighting : The Netherlands [online]. August 2002, [cited 2010-10-02]. Available from

z WWW: <www.lighting.philips.com/in_en/applications/industry/pdf/industrial_lighting_and_productivity/pli-

0005_whitep-uk_20sep.pdf>.

[4] HOLLWICH, F.: The Influence of Ocular Light Perception on Metabolism in Man and in Animal. Springer-Verlag, New York, 1979, 129 p., ISBN 0387903151.

[5] GOOLEY, J. J. et al.: Spectral Responses of the Human Circadian System Depend on the Irradiance and Duration of Exposure to Light. Science, Translational Medicine [online]. 2010-05-12, 2010, issue 31 [cited 2010-10-02], ISSN 1946-6242, DOI, 10.1126/scitranslmed.3000741.

[6] FIGUEIRO, M.: Research matters: measure up for healthy lighting. LD+A., 2005, vol. 35, no. 1, pp. 14-16, ISSN 0360-6325.

[7] HUBALEK, S.: LuxBlick: Messung der täglichen Lichtexposition zur Beurteilung der nicht-visuellen Lichtwirkungen über das Auge. Shaker Verlag, Zürich, 2007, 221 s. Disertační práce, ETH Zürich. Dostupné z WWW: <http://e-collection.ethbib.ethz.ch/view/eth:29804>. DOI:10.3929/ethz-a-005429531.

[8] BOMMEL VAN, W. J. M. -BELD VAN DEN, G. J.: Lighting for work: a review of visual and biological effects. Lighting Research and Technology, December 2004, 36, 4, pp. 255-266, ISSN 1477-1535,DOI:10.1191/1365782804li122oa.

[9] INTERNATIONAL DARK-SKY ASSOCIATION: Achievements in High Brightness White LEDs. Specifier Bulleting for Dark Sky Applications [online], 2010, Volume 3, Issue 1 [cit. 2010-10-02]. Dostupnýz WWW: <http://docs.darksky.org/SB/LED-SB-v3i1.pdf>.

[10] GALL, D. - LAPUENTE, V.: Beleuchtungsrelevante Aspekte bei der Auswahl eines förderlichen Lampenspektrums, Teil 1, Teil 2

[cit. 2010-10-02]. Dostupný z WWW: <http://www.tu-ilmenau.de/fileadmin/public/lichttechnik/Publikationen/2003/teil1.pdf; teil2.pdf>.

[11] PRAŠKO, J –BRUNOVSKÝ, M. – ZÁVĚŠICKÁ, L.: Fototerapie a její indikace. Psychiatrické centrum, 3. LF UK, Praha [online], 2006 [cit. 2010-10-02]. Dostupný z WWW: <http://www.tigis.cz/PSYCHIAT/psychsupp3_05/14_Prasko.htm>.

Review: MUDr. Milena Jirásková, Department of Skin Clinic, 1st Faculty of Medicine, Charles University in Prague

Author. Antonín Fuksa
Published in Světlo 6/2010