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Scientific research has repeatedly found evidence that prolonged exposure to blue light in the evening and at night can promote a wide range of chronic diseases, such as cardiovascular disease, diabetes and certain types of cancer.
As with all chronic diseases, it is challenging to identify and substantiate a clear cause, as these diseases often develop over a period of many years. Using the well-studied example of breast cancer, we show here how researchers arrive at robust results based on circumstantial evidence.
Because of the known hormonal effects of light at night, hormone-dependent cancers such as breast cancer are a frequent subject of research. In a well-reviewed study (“Light at night co-distributes with incident breast but not lung cancer in the female population of Israel“), breast cancer incidence was correlated with light exposure at night, as assessed by satellite data.
After accounting for other factors such as income and ethnic constitution of the population, the communities with the highest nighttime light exposure were calculated to have a 73% higher risk of breast cancer. The unchanged lung cancer risk served as a “control value” because lung cancer is assumed to be non-hormone dependent, and therefore the risk is independent of nocturnal light exposure.
This compelling evidence was the origin of a fascinating review study:
While the above study is compelling evidence that nighttime light may lead to higher breast cancer rates, it is not conclusive proof. Because the high road to medical evidence, the double-blind, placebo-controlled trial, is difficult to conduct with nighttime light (what would one use as a light placebo?), the plausibility of the theory can only be tested by a meta-analysis of existing studies (“Light-at-night, circadian disruption and breast cancer: assessment of existing evidence“).
The following correlations would be required to be found if nocturnal light exposure indeed increases breast cancer risk:
1. female shift workers (night shift) should have a higher risk
2. blind women should have a lower risk
3. sleep duration (as a marker for hours of darkness) should be indirectly proportional to risk
4. the amount of light in the bedroom would have to be directly proportional to the risk
5. population studies would show an association between urban lighting levels and breast cancer risk.
The meta-analysis concludes that all of these assumptions can be demonstrated in medical studies.
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