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.