Does light have a dark side?: nighttime illumination might elevate cancer risk

Science News, Oct 17, 1998 by Janet Raloff

In the March MOLECULAR CARCINOGENESIS, they lay out the molecular basis for concerns that light at night alight prove an endocrine disrupter with the potential to increase cancer risk. They note that recent findings in several laboratories working with cells and with tissues removed from animals indicate that a reduction of melatonin can alter the production of other hormones, may suppress the immune system's ability to recognize and respond to newly emerging cancers, and appears to spur the growth of at least some tumor tissues.

By studying which cells possess melatonin receptors and how cells use them to respond to the hormone--as Barrett's team and others are now doing--science may resolve whether nighttime illumination truly threatens health, and if so, how much and in whom.

Studies now under way are also testing which wavelengths--or colors--are most biologically active. For instance, blue and green light appear especially effective at inhibiting melatonin synthesis in healthy young men, according to studies by Brainard. Indeed, he notes that for some colors, "17 lux was sufficient to produce strong melatonin suppression in these men--and some had full suppression with exposure to as little as 5 lux." The latter "is a little more illumination than what you'd have with full moonlight."

Brainard notes that the payoff for finding out what wavelengths are most hormonally disruptive could be insights on how "to tailor nighttime lighting to provide good vision without interfering with the melatonin rhythm." He says that "it may also help us develop more effective lights for use in treating winter depression and sleep disorders."

RELATED ARTICLE: How much light is too much?

At least in rats, a little light throughout the night can have a dramatic impact on-s cancer, observes David E. Blask of the Mary Imogene Bassett Research Institute in Cooperstown, N.Y.

Tumors can grow especially rapidly in rodents exposed to constant light, he notes--presumably because of a near-total suppression of their melatonin. To test just how much light was necessary to enhance tumor growth, he implanted liver-cancer cells into rats.

His team housed one group of caged animals in a room illuminated around-the-clock with about 850 lux of white light, which is roughly equivalent to an office with medium lighting. A second group of animals spent their days in 850 lux but their nights in total darkness. A third group encountered almost the same light-dark cycle. The only difference: 0.2 lux leaked in at the bottom of the door to their room from a hallway outside--illumination well below that typical of a moonless night, he says.

In the October 1997 LABORATORY ANIMAL SCIENCE, Blask's team reported that tumors in animals exposed to the crack of light coming under the room's door grew almost twice as fast as those in animals getting a night of total darkness. Indeed, he says, "animals exposed to the low-level light contamination had a tumor-growth rate virtually identical to that in the animals exposed to bright, constant light." He has just replicated the findings in an experiment in which the lighting was more rigorously controlled.