Although our bodies evolved to work while the sun shines and to rest at night, people in today’s 24/7 society sleep, work, and play with little regard for solar cycles. This flaunting dominion over darkness may come at a cost, however—a heightened risk of cancer.
Two dozen scientists from 10 countries met in Lyon, France under the aegis of the International Agency for Research on Cancer, last fall to review human and animal data on health risks from exposures to light at night. The scientists now report that six of eight epidemiological studies—”most notably two independent cohort studies of nurses engaged in shift-work at night”—exhibited a “modestly increased risk of breast cancer in long-term employees” when compared to people who did not work at night.
The presumed mechanism, the panel noted, is that nighttime illumination shuts down nocturnal secretion of melatonin. This hormone, normally produced in darkness (SN: 5/13/95, p. 300), appears to possess anticancer properties (SN: 1/7/06, p. 8).
Despite “limited” human data, the IARC panel found “sufficient” evidence from animal experiments to conclude that nighttime shift work “is probably carcinogenic to humans.” The panel previewed its findings in the December 2007 Lancet Oncology and plans to publish a detailed follow-up in an IARC monograph.