Janine O’Leary Cobb

On January 12th, 2002, the Globe & Mail ran a long article entitled “Blinded by the light” covering the the ramifications and repercussions of a world flooded by artifical light. Included was a short report on the work of Richard Stevens, an epidemiologist at the University of Connecticut, who is working on the idea that light at night is linked to, and perhaps responsible for, increasing rates of breast cancer. This is breast cancer linked to environmental conditions but perhaps not in the usual way that we have contemplated (i. e., high fat diets, use of alcohol, exposure to toxic chemicals, etc.).

The hypothesis that light at night might be a potential risk factor for breast cancer was first raised ten years ago by Stevens and his fellow researchers in an article, “Electric power, pineal function, and the risk of breast cancer.” Since then, research has diversified to include studies of breast cancer in the blind; studies of the hormone, melatonin, and its possible effect on breast cancer; and studies of the effects of shift work on human health.

These areas are not so disparate as they may seem.

The notion that artificial light might have a deleterious effect comes from a number of sources. One is the very high rate of breast cancer in industrialized countries as compared with less developed countries. Another is the fact that blind women have rates of breast cancer reported to be somewhere between four-fifths to one-half those of sighted women. A third is yielded by research into the effects of melatonin.

Melatonin is a hormone made and secreted by the pineal gland situated deep within the brain. Until the mid-1950s it was widely taught that the pineal gland was a non-functioning brain structure. Then it was discovered that the pineal gland produced a number of neuropeptides including melatonin, a hormone generated only in the dark and suppressed by light. In humans, malatonin secretion starts around sunset, peaks in the middle of the night, and falls off towards dawn. Any bright light inhibits melatonin secretion but any kind of light during sleep—even a nightlight—can diminish the production of melatonin.

There was a surge of interest in melatonin some years ago, based on its purported ability to modify the effects of jet lag and alleviate insomnia. There was—and continues to be—enormous disagreement about the amounts of melatonin that can readjust sleep/wake cycles (i. e., circadian rhythms), the extent of its effects (elevated melatonin is apparently characteristic of both depression and SAD—seasonal affective disorder), and the optimal time to take it. Melatonin is classed as a hormone in Canada, and thus available only prescription. However, it is easily available in the States and through sites on the internet (although there is no guarantee of the quality of the product).

Melatonin production is almost twice as high in youth as compared to old age. According to an article that Sharon Batt wrote for Breast Cancer Action San Francisco’s Newsletter, “As we age, calcium deposits accumulate in the pineal gland... and both melatonin levels and hours of sleep may be diminished as a result.” Knowing that risk of breast cancer increases with age, it is interesting to speculate about whether the concomitant reduction in melatonin is simply a correlation, or if some causality is involved.

This kind of speculation has led a number of researchers to look at the effects of shift work. If women are forced to sleep during daylight hours, will this affect their risk of breast cancer?

Last October 17th, two studies were published in the Journal of the National Cancer Institute. One was a case control study of Seattle residents diagnosed with breast cancer, matched against controls. The other was an analysis of new data regarding shift work from the Nurses’ Health Study. Both studies were adjusted to account for reproductive history, a family history of breast cancer, use of oral contraceptives and hormone replacement therapy, social class and alcohol consumption. Both concluded that sleep interruption, especially in women working the graveyard shift, is associated with increased risk.

In an editorial in the same issue of the JNCI, Johnni Hansen of the Danish Cancer Society commented that the two studies were complementary and constituted important contributions to the search for causes of breast cancer. If the reports of breast cancer rates among blind women are correct, it becomes even more plausible.

What to do? We are obviously not about to sleep and rise with the sun as our ancestors did—despite the fact that many of us know how health-giving this feels when camping. Because even a sliver of light (one researcher mentioned the light that comes in under a hotel-room door) can make a difference, the darker the bedroom the better. Eye shades might be an idea.

Perhaps the next step is a study that extends the research into the brightness level of our bedrooms (the Seattle study found that women with the brightest bedrooms had an increased risk) and compares breast cancer rates among women who stay up until midnight or later every night, with the early-bird types who are in bed by 9. I can’t wait to see the results.

References:

• Batt, S. What light through yonder window wreaks: Circadian rhythms and breast cancer. Breast Cancer Action San Franciso Newsletter, September 2000.
• Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. JNCI 2001; 93 (20): 1557-1562
• Hansen, J. Editorial: Light at night, shiftwork, and breast cancer risk. JNCI 2001; 93(20): 1513-1515
• Mittelstaedt, M. Blinded by the light. Globe & Mail, Jan. 12, 2002.
• Rajaratnam SM, Arendt J. Health in a 24-hour society. Lancet 2002, 358 (9286): 999-1005
• Schernhammer ES, Laden F, Speizer FE et al. Rotating night shifts and risk of breast cancer in women participating in the Nurses’ Health Study. JNCI 2001; 93 (20): 1563-1568
• Stevens RG, Davis S, Thomas DB et al. Electric power, pineal function, and the risk of breast cancer. FASEB J 1992; 6:853-860