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Fig. 1: Daily rhythms of the human body. Fig. 2: C1,2 —possible shapes of relative sensitivity of Circadian sensor, smel—the 2015 Melanopic action spectrum and V() – the spectral luminous efficiency function. Introduction Temporal rhythms can be found throughout the whole natural world. The circadian (from Latin circa—around, diem—day) have been discussed in the magazine Svetlo (Czech lighting journal) several times in a past few years. The expression ‘circadian’ was introduced in the 1950s by Franz Halberg, one of the founders of chronobiology, the science of temporal order in the living realm. From Czech chro- nobiologists, Prof Helena Illnerová and her team are the most well-known for the discovery of melatonin secretion variation in rats, which depends on light exposurechangesoverthefourseasons,aswellasfor the discovery of photosensitivity in the biological clock in suprachiasmatic nuclei in hypothalamus.1,2 In the autonomous nervous system of mammals, the central biological clock is found under the cross- ing of optical nerves in the suprachiasmatic nuclei (SCN), which controls hormone levels in blood, body temperature, sleep and alertness, to name just a few. Melatonin is the hormone governing sleep and body regeneration, while cortisone is the hormone con- nected with activity, stress and motion. Examples of levels observed, courtesy of Philips Lighting, are shown in Fig. 1.3 The patterns of the curves vary slightly each day. The central biological clock is synchronised by light, but food intake also matters. In young hu- mans, this clock has a circa of 24 hours when running free of light synchronisation, which is the origin of the term 'circadian'. Individual organs have their local clock synchronised with the SCN ‘master’ clock. Light is the strongest synchroniz- ing agent (Zeitgeber). A dose of several lux of suitable spectral distribution for several minutes can already cause level of melatonin in blood to decrease. Effects of light on living organisms Prof.FritzHollwich,anauthorofanophthalmology textbook and inventor of many procedures in oph- thalmology, has studied these effects closely. In his inaugural dissertation from 1948, he distinguishes the visual and energetic (non-visual) function of the eye. He found that patients suffering blindness due to cataracts had different levels of certain hor- mones and other markers in the blood, compared to the normally sighted population. When the patients regained their sight after an operation, the levels returned to normal. He also found that some distri- butions of light, lack of light or excess light or its invariance have adverse effects on organisms. In the last few years, a novel photoreceptor—intrinsic pho- tosensitive retinal ganglion cells (ipRGCs) is often discussed.Thesehadalreadybeenfoundinmiceback in 1991 and in humans as late as 2007. ipRGCs con- tain melanopsin photo pigment, which maximum sensitivity is reported between 450–482 nm (rarely | feature wellbeing 28 cosmetic dentistry 1 2016 Author: Antonín Fuksa, Czech Republic Fig. 1 Fig. 2 Light and the biological clock © Viktorija Reuta dentistry 12016