After them, Homo erectus started to use fire in caves around 500,000 years ago. The first evidence arises from attempts to control fire by Australopithecus 1.42 million years ago. Humans have been using artificial light for many years. The visible spectrum includes the range from 390 to 780 nanometers (nm), with a peak sensitivity at 555 nm. Light is a wave corresponding to a small part of the electromagnetic spectrum to which human eyes are responsive. This review addresses the state of the art regarding the crosstalk between light and the circadian system. Thus, the development of lighting systems that preserve the melatonin rhythm could reduce the health risks induced by chronodisruption. Nocturnal blue light exposure is currently increasing, due to the proliferation of energy-efficient lighting (LEDs) and electronic devices. Blue light, which is particularly beneficial during the daytime, seems to be more disruptive at night, and induces the strongest melatonin inhibition. Importantly, not all wavelengths of light are equally chronodisrupting. Knowledge of retinal photoreceptors and the discovery of melanopsin in some ganglion cells demonstrate that light intensity, timing and spectrum must be considered to keep the biological clock properly entrained. Epidemiological studies show that CD is associated with an increased incidence of diabetes, obesity, heart disease, cognitive and affective impairment, premature aging and some types of cancer. In spite of the positive impact of artificial light, we pay a price for the easy access to light during the night: disorganization of our circadian system or chronodisruption (CD), including perturbations in melatonin rhythm. Currently, in developed countries, nights are excessively illuminated (light at night), whereas daytime is mainly spent indoors, and thus people are exposed to much lower light intensities than under natural conditions.
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February 2023
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