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

Science News,  Oct 17, 1998  by Janet Raloff

• E-mail
• Print
• Link

Steven W. Lockley, a chronobiologist at the University of Surrey in England, and his colleagues have made such measurements. And in the November 1997 JOURNAL OF CLINICAL ENDOCRINOLOGY AND METABOLISM, they noted that melatonin cycles in the blind are anything but predictable.

His group recruited 49 legally blind individuals to participate in a roughly month-long trial. Each collected his or her urine over a 48-hour span each week. The scientists then measured a melatonin byproduct in the urine.

Among the 30 people unable to perceive light, 57 percent had a free-running rhythm--a cycle longer or shorter than 24 hours. "This included every single subject that we've studied Who has had their eyes removed," he notes. Another 23 percent had a normally cycling clock, with melatonin reliably peaking at night. The remainder had abnormal or unclassifiable cycles.

Even among the 19 people in the study who could perceive light, 26 percent exhibited abnormal rhythms, with melatonin production peaking at times other than the middle of the night.

"So one can't assume that the melatonin rhythm in all blind people . . . is free-running--or that its peaks in lightsensitive individuals will be normal," observes neuroendocrinotogist David E. Blask of the Mary Imogene Bassett Research Institute in Cooperstown, N.Y.

Blask's studies of rats suggest that an abnormal timing of melatonin peaks can have a powerful effect on cancer.

He administered cancer-causing chemicals to rats and then over subsequent weeks injected the animals daily with melatonin. The injections were timed to produce peaks during daylight hours, when melatonin concentrations should have been negligible.

When those injections occurred midmorning, tumors grew at the same rates seen in animals not receiving injections. However, in animals that received the hormone during the afternoon, "we see an inhibitory effect of the hormone on tumor growth, not only in liver cancers, but also in breast cancers."

The findings suggest "that there is a rhythm of sensitivity within tumor tissues or in cells susceptible to becoming tumors," he told SCIENCE NEWS. "And maybe in people who can't perceive light, the oscillating cycle of their biological clock causes their melatonin peaks to coincide with the inhibitory period of tumor cells more often than they do in light-sensitive people."

The growing body of data on melatonin, light, and cancer suggests that certain populations, such as shift workers or others who regularly work in bright light at night, could face unusual risks, Blask argues. Certainly, he says, "the data are suggestive enough to raise eyebrows and prompt further serious study."

William S. Baldwin and J. Carl Barrett of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., agree that such theories should be tested--by probing the most likely mechanisms of light's effects. "When the melatonin hypothesis was first presented," the pair notes, "no putative melatonin receptors [on cells] were known." Since then, three types have been identified.