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

Science News, Oct 17, 1998 by Janet Raloff

Since life began, one pattern has dominated Earth's natural environment--a daily rhythm of intense sunlight alternating with nights of near total darkness. As a source of heat and energy, sunlight powers a majority of the planet's biological activities. When that light disappears, much of the world rests.

Humans, the grand manipulators, have not been content to cede control of their activity cycle to the heavens, however. People have spent eons developing ever better means to artificially extend the day. Thanks to widespread electrification and color-corrected, high-watt lightbulbs, synthetic sunlight can now bombard city dwellers around-the-clock.

This attempt to erase the night--or at least to confine it to small, artificially defined windows--may come with a price. At a minimum, it can lead to a chronic lack of sleep, diminishing the effectiveness of the body's immune system. Some new studies, however, suggest the possibility of an even more worrisome threat.

Exposure to light at night can disrupt the body's production of melatonin, a brain hormone best known for its daily role in resetting the body's biological clock (SN: 5/13/95, p. 300). Secreted primarily in the brain, and at night, melatonin triggers a host of biochemical activities, including a nocturnal reduction in the body's production of estrogen. Some researchers have speculated that chronically decreasing nocturnal melatonin production--as with light--might increase an individual's risk of developing estrogen-related malignancies, such as breast cancer.

Two studies in Nordic populations now offer tentative support for this idea.

According to neuroendocrinologist Russel J. Reiter of the University of Texas Health Science Center at San Antonio, the emerging science indicates that, functionally, "light is a drug"--and that "by abusing it, we risk imperiling our health."

Light entering the eye allows our brains to sense the shape, size, color, and motion of objects around us. It also summons, albeit imperceptibly, a cadre of other biological sentinels. These go on to trumpet light's presence to distant tissues--organs and cells lacking the means to detect illumination directly.

When these biochemical fanfares occur late at night, they can alter the timing of melatonin's peak output, as a landmark study in 1980 showed. Alfred J. Lewy and his colleagues at the National Institute of Mental Health in Bethesda, Md., shut down melatonin production in men by waking and exposing them to 2,500 lux of white light at 2 a.m., when synthesis of the hormone was at its peak. (For perspective, 100 lux may be found in a comfortably dim living room, whereas sunlight at high noon on a cloudless day can blast the eyes with 100,000 lux.) At the Oregon Health Sciences University in Portland, 8 years later, Lewy and George C. Brainard, now at Thomas Jefferson University in Philadelphia, found that just 50 lux could do the same trick--if it is green light.

At about the time this work was going on, Richard G. Stevens of the Energy Department's Pacific Northwest National Laboratory in Richland. Wash., was developing a controversial theory now known as the melatonin hypothesis. It holds that longterm environmental perturbations in natural rhythms of melatonin secretion--by exposure to electromagnetic fields (SN: 1/10/98, p. 29) or to light at night--might increase cancer risk, especially in the breast, by increasing estrogen exposure.

Since the theory's debut, researchers have shown in animals that melatonin also functions as an antioxidant (SN: 8/14/93, p. 109) and an anticarcinogen. Some rodent studies have also demonstrated that certain nascent cancers grow more rapidly when the animals encounter even low levels of light at night (see How much light is too much?)

The first preliminary evidence linking light to cancer in people emerged 8 years ago in a report by Robert A. Hahn of the Centers for Disease Control and Prevention in Atlanta. After combing statistics from a national survey on women who had been hospitalized between 1979 and 1987--including some 11,700 with breast cancer--he computed the incidence of this malignancy in blind and sighted women. If light alters cancer risk through some disruptive effect on melatonin, the epidemiologist reasoned, people whose eyes can't detect light should prove resistant. As a further test he looked at heartdisease incidence, where melatonin should play no role.

In the May 1991 EPIDEMIOLOGY, Hahn reported that although the profoundly blind women proved as likely as the sighted women to get heart disease, they appeared only half as prone to develop breast cancer.

Probing this idea in more detail, Maria Feychting and her colleagues at the Karolinska Institute in Stockholm have just compared cancer incidence in 1,600 profoundly blind men and women with that in 13,000 people having severe visual impairment. Because members of the second group could still perceive light, Feychting explains, they should resemble sighted people in terms of any light effects on melatonin.