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Erren, T. C., & Reiter, R. J. (2009). Light Hygiene: Time to make preventive use of insights--old and new--into the nexus of the drug light, melatonin, clocks, chronodisruption and public health. Med Hypotheses, 73(4), 537–541.
Abstract: Light is, clearly, a key to life on Earth and light, equally clearly, determines biological rhythmicity in organisms. Light does the latter by setting internal or endogenous clocks which allow a multitude of species, including man, to adjust their lives to changing external or environmental conditions. Critical changes over time occur from day to night and throughout the year. In this paper, we sum up how visible light provides electromagnetic information about environmental “time” via the ocular interface of newly discovered photoreceptive cells to a master clock in our brain, viz the suprachiasmatic nuclei [SCN], and how the SCN translate this input, with melatonin as a key biologic intermediary, into endogenous or biological time. We summarize experimental and epidemiological evidence suggesting how chronodisruption, a relevant disturbance of the temporal organization or order of physiology, endocrinology, metabolism and behaviour, is probably detrimental for human beings. On the basis of our synthesis, and in line with suggestions by other researchers voiced decades ago, light must, functionally, be considered as a drug equivalent. In this vein, the very timing, quality (wavelength), quantity (dose) and side effects, including chronodisruption, of light exposures can be critically important for health and disease in man. As a promising means to foster public health, we advocate an appropriate balance of exposures to the key Zeitgeber light in terms of “light hygiene”, implying strong and appropriate rather than weak and confusing temporal information. This focus on “light hygiene”, and thus on the key Zeitgeber light, does not mean to ignore that there are multiple entrainment pathways for our circadian clocks. Indeed, when dealing with light, chronodisruption and a multitude of adverse health effects, we ultimately need to consider Zeitgeber cues, and their possible interplay, beyond light alone. Confusions of the temporal programmes in humans can also stem from physical and social activities, stress and facets of food intake. And yet, since light possesses a rather unique and exclusive Zeitgeber role and in view of its ubiquitous nature, a specific, preventative focus on “light hygiene”, as a contribution to a general “Zeitgeber hygiene”, is warranted.
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Meng, Y., He, Z., Yin, J., Zhang, Y., & Zhang, T. (2011). Quantitative calculation of human melatonin suppression induced by inappropriate light at night. Med Biol Eng Comput, 49(9), 1083–1088.
Abstract: Melatonin (C(1)(3)H(1)(6)N(2)O(2)) has a wide range of functions in the body. When is inappropriately exposed to light at night, human circadian rhythm will be interfered and then melatonin secretion will become abnormal. For nearly three decades great progresses have been achieved in analytic action spectra and melatonin suppression by various light conditions. However, so far few articles focused on the quantitative calculation of melatonin suppression induced by light. In this article, an algorithm is established, in which all the contributions of rods, cones, and intrinsically photosensitive retinal ganglion cells are considered. Calculation results accords with the experimental data in references very well, which indicate the validity of this algorithm. This algorithm can also interpret the rule of melatonin suppression varying with light correlated color temperature very well.
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Pauley, S. M. (2004). Lighting for the human circadian clock: recent research indicates that lighting has become a public health issue. Med Hypotheses, 63(4), 588–596.
Abstract: The hypothesis that the suppression of melatonin (MLT) by exposure to light at night (LAN) may be one reason for the higher rates of breast and colorectal cancers in the developed world deserves more attention. The literature supports raising this subject for awareness as a growing public health issue. Evidence now exists that indirectly links exposures to LAN to human breast and colorectal cancers in shift workers. The hypothesis begs an even larger question: has medical science overlooked the suppression of MLT by LAN as a contributor to the overall incidence of cancer? The indirect linkage of breast cancer to LAN is further supported by laboratory rat experiments by David E. Blask and colleagues. Experiments involved the implanting of human MCF-7 breast cancer cell xenografts into the groins of rats and measurements were made of cancer cell growth rates, the uptake of linoleic acid (LA), and MLT levels. One group of implanted rats were placed in light-dark (12L:12D) and a second group in light-light (12L:12L) environments. Constant light suppressed MLT, increased cancer cell growth rates, and increased LA uptake into cancer cells. The opposite was seen in the light-dark group. The proposed mechanism is the suppression of nocturnal MLT by exposure to LAN and subsequent lack of protection by MLT on cancer cell receptor sites which allows the uptake of LA which in turn enhances the growth of cancer cells. MLT is a protective, oncostatic hormone and strong antioxidant having evolved in all plants and animals over the millennia. In vertebrates, MLT is normally produced by the pineal gland during the early morning hours of darkness, even in nocturnal animals, and is suppressed by exposure to LAN. Daily entrainment of the human circadian clock is important for good human health. These studies suggest that the proper use and color of indoor and outdoor lighting is important to the health of both humans and ecosystems. Lighting fixtures should be designed to minimize interference with normal circadian rhythms in plants and animals. New discoveries on blue-light-sensitive retinal ganglion cell light receptors that control the circadian clock and how those receptors relate to today's modern high intensity discharge (HID) lamps are discussed. There is a brief discussion of circadian rhythms and light pollution. With the precautionary principle in mind, practical suggestions are offered for better indoor and outdoor lighting practices designed to safeguard human health.
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Wyse, C. A., Selman, C., Page, M. M., Coogan, A. N., & Hazlerigg, D. G. (2011). Circadian desynchrony and metabolic dysfunction; did light pollution make us fat? Med Hypotheses, 77(6), 1139–1144.
Abstract: Circadian rhythms are daily oscillations in physiology and behaviour that recur with a period of 24h, and that are entrained by the daily photoperiod. The cycle of sunrise and sunset provided a reliable time cue for many thousands of years, until the advent of artificial lighting disrupted the entrainment of human circadian rhythms to the solar photoperiod. Circadian desynchrony (CD) occurs when endogenous rhythms become misaligned with daily photoperiodic cycles, and this condition is facilitated by artificial lighting. This review examines the hypothesis that chronic CD that has accompanied the availability of electric lighting in the developed world induces a metabolic and behavioural phenotype that is predisposed to the development of obesity. The evidence to support this hypothesis is based on epidemiological data showing coincidence between the appearance of obesity and the availability of artificial light, both geographically, and historically. This association links CD to obesity in humans, and is corroborated by experimental studies that demonstrate that CD can induce obesity and metabolic dysfunction in humans and in rodents. This association between CD and obesity has far reaching implications for human health, lifestyle and work practices. Attention to the rhythmicity of daily sleep, exercise, work and feeding schedules could be beneficial in targeting or reversing the modern human predisposition to obesity.
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