A new study may explain why female night-shift workers have a higher rate of breast cancer than other women and firm-up the link between the hormone melatonin and cancer. The study may also offer a new explanation for the epidemic rise in breast cancer in industrialized countries. "This is the first experimental evidence that artificial light plays an integral role in the growth of human breast cancer," said National Institute of Environmental Health Sciences Director David A. Schwartz. "This finding will enable scientists to develop new strategies for evaluating the effects of light and other environmental factors on cancer growth."
The study, published in the journal Cancer Research, found that artificial light stimulated the growth of human breast tumors by suppressing the levels of a key hormone called melatonin. The results also showed that extended periods of nighttime darkness greatly slowed the growth of these tumors. Research in the past showed that artificial light suppresses the production of melatonin, a hormone that helps to regulate a person's sleeping and waking cycles.
"The risk of developing breast cancer is about five times higher in industrialized nations than it is in underdeveloped countries," the researchers said. "These results suggest that the increasing nighttime use of electric lighting, both at home and in the workplace, may be a significant factor."
Melatonin influences the body's circadian rhythm, an internal biological clock that regulates the sleep-wake cycle, body temperature and endocrine functions. "Evidence is emerging that disruption of one's circadian clock is associated with cancer in humans, and that interference with internal timekeeping can tip the balance in favor of tumor development," said researcher David Blask.
"The effects we are seeing are of greatest concern to people who routinely stay in a lighted environment during times when they would prefer to be sleeping," added co-researcher Mark Rollag. "This is because melatonin concentrations are not elevated during a person's normal waking hours."
"If the link between light exposure and cancer risk can be confirmed, it could have an immediate impact on the production and use of artificial lighting in this country," concluded Blask. "This might include lighting with a wavelength and intensity that does not disrupt melatonin levels and internal timekeeping. Companies that employ shift workers could introduce lighting that allows the workers to see without disrupting their circadian and melatonin rhythms."
Source: NIH/National Institute of Environmental Health Sciences
