No scientific publications currently address the relationship between light intensity specifically from incandescent light bulbs and the incidence of cancer or cancer-like effects. However, over the past few years there have been many studies on the potentially carcinogenic effects of night work, shift work, artificial light at night and sleep deprivation. There is significant evidence that people exposed to one or more of these factors have an increased risk of developing certain types of tumours, such as breast cancer. But the specific source of the light, whether tungsten light bulbs or fluorescent tubes, has not yet been assessed.
Scientists do not yet know the precise biological reasons why artificial light, etc. can increase cancer risks. The International Agency for Research on Cancer (IARC) found that shift work that involves circadian disruption (unnatural day/night biorhythms) is probably carcinogenic to humans. Its assessment is, however, primarily based on the results of animal experiments. The only statement that can be definitively made with regard to humans is that if we are exposed to light during the night, our melatonin production is immediately reduced. The extent of this reduction depends on the intensity of the light, the wavelength, and the duration of exposure. Further studies are required to identify the exact influence that lighting intensity during night work and shift work has on our risk of developing cancers. Many scientists assume that an accumulation of the risk factors to which shift workers are exposed may promote cancer development. These would include secondary factors such as an unhealthy diet, higher smoking rates, a higher level of alcohol consumption, and a lack of exercise.
Dr Beatrice Kunz of the Krebsinformationsdienst at the German Cancer Research Center answered this question.