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Breast cancer (BC) is increasing worldwide together with light pollution (LP) emerging from various outdoor and indoor sources. Results of different studies including our research centre report on the relations between BC-incidences and exposure to Artificial Light at Night (ALAN). The trend for energy saving-ALAN increases the problem, as light intensity is increasing and mainly that of short wavelength (SWL), within the blue part of the spectrum (450–500 nm). Our master biological clock, located in the hypothalamus, entrained by light/dark cycles is in charge of our temporal organisation from cell functions. It is not only light-intensity, which changes with the 24 hour cycles, but also the dominant parts of the light spectrum, which reach’s earth. Those dominant parts, signalling for daytime are the SWL, including those between 450–500 nm a range known as an efficient suppressor of the nocturnal pineal produced hormone Melatonin (MLT). We attempted to study the nexus: ALAN, MLT-Suppression, epigenetic modifications and BC-cells proliferation in subcutaneously inoculated female mice.Mice were acclimated for two weeks under 8L:16D, at a constant ambient temperature testing various sources of illumination differing in spectrum composition. After inoculation, we exposed mice to ALAN of the same illumination of daytime. We measured the following variables: Body mass, tumour volume, MLT-production and levels of Global DNA methylation (GDM) levels.We revealed the existence of the proposed nexus. Response to ALAN is depended on the wavelength illumination source. SWL-illumination bulbs as white-LED or compact florescent have a higher negative effect compared with that of incandescent or carbon bulbs. We emphasise a relation between tumour volume, level of MLT-suppression and GDM-levels.We suggest that human populations under increasing LP-levels of SWL-illumination are in a high risk for becoming BC-patients, it should be of great interest to set the threshold for exposure to SWL-illumination and BC-risk.