A study of NASA satellite data, overlaid with reported cancer statistics, has identified nighttime exposure to lighted areas as a risk factor for breast cancer:
Women who live in neighborhoods with large amounts of nighttime illumination are more likely to get breast cancer than those who live in areas where nocturnal darkness prevails, according to an unusual study that overlaid satellite images of Earth onto cancer registries.
"By no means are we saying that light at night is the only or the major risk factor for breast cancer," said Itai Kloog, of the University of Haifa, who led the new work. "But we found a clear and strong correlation that should be taken into consideration."
The mechanism of such a link, if real, remains mysterious, but many scientists suspect that melatonin is key.
A tumor suppressing hormone long known to be impacted by the nighttime illumination, melatonin requires darkness for its synthesis and release:
Melatonin is a neurohormone produced in the brain by the pineal gland, from the amino acid tryptophan. The synthesis and release of melatonin are stimulated by darkness and suppressed by light, suggesting the involvement of melatonin in circadian rhythm and regulation of diverse body functions.
(The study has not recommended melatonin supplementation. Not enough is known about the melatonin connection. Further studies will be required).