The researchers then took blood samples from 12 healthy, premenopausal volunteers. The samples were collected under three different conditions -- during the daytime, during the nighttime following 2 hours of complete darkness, and during the nighttime following 90 minutes of exposure to bright fluorescent light. These blood samples were then pumped directly through the developing tumors.

"The melatonin-rich blood collected from subjects while in total darkness severely slowed the growth of the tumors. "These results are due to a direct effect of the melatonin on the cancer cells," said Blask. "The melatonin is clearly suppressing tumor development and growth."


In contrast, tests with the melatonin-depleted blood from light-exposed subjects stimulated tumor growth. "We observed rapid growth comparable to that seen with administration of daytime blood samples, when tumor activity is particularly high," Blask said.

According to the researchers, melatonin exerts a strong influence on the body's circadian rhythm, an internal biological clock that regulates sleep -- wake cycle, body temperature, endocrine functions, and a number of disease processes including heart attack, stroke and asthma. "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 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," said Mark Rollag, Ph.D., a visiting research scientist at the University of Virginia and one of the study co-authors. "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," said Richard Stevens, Ph.D., an epidemiologist with the University of Connecticut Health Center who has authored several papers on the subject. "This might include lighting with a wavelength and intensity that does not disrupt melatonin levels and internal timekeeping."

"Day workers who spend their time indoors would benefit from lighting that better mimics sunlight," added Stevens. "Companies that employ shift workers could introduce lighting that allows the workers to see without disrupting their circadian and melatonin rhythms."

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