The Braille Monitor April 2005
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and Sleep Survey of Blind Women
by Steven W. Lockley, Ph.D.
From the Editor: The NFB has agreed to work with Brigham and Women's Hospital and Harvard Medical School to gather data that may shed light on breast cancer risk. Here is the information:
Over the past fifty years the incidence of breast cancer has risen dramatically in the U.S. Similar increases have also been seen in countries that have recently developed Western-style economies. The explanation for these increases is in doubt, although family history, diet, medication, smoking, alcohol use, and reproductive history can account for about 50 percent of the known risk. The search for other factors associated with modern living that relate to breast cancer risk is ongoing in order to identify those which are avoidable, which can be altered by providing health and lifestyle advice to women.
One notable factor in modern society is the use of artificial electric light, which has eliminated much of our natural exposure to darkness. Light exposure at night has effects on the human body such as stimulating the brain to be more alert and changing the timing of the internal twenty-four-hour body clock. Light exposure at night also stops the production of a hormone called melatonin, which occurs only at night. Exposure to even room-level lighting is enough to stop melatonin production, and brighter lights or longer exposure causes more suppression. The effects of stopping melatonin production at night are unknown. Many animal studies, however, suggest that melatonin may slow down growth, particularly in breast cancer. Such studies have not been conducted in people, and the way melatonin relates to cancer risk in humans is not known. If melatonin production does affect cancer risk, we may expect several groups who are regularly exposed to bright light for long durations at night to have more cancer. Based on this theory, several U.S. and European studies have investigated female shift workers who are exposed to light at night and have found a 36-60 percent increase in breast cancer risk in women working night shifts regularly over many years. Similar findings have been made in surveys of female flight attendants who are also exposed to light at night when traveling across time zones.
The theory suggests that women who are not exposed to light at night may be at a reduced risk of breast cancer. This hypothesis has been examined in several studies of cancer risk in visually impaired women. These studies have shown that visually impaired women are at a reduced risk of breast cancer compared to sighted women and that reduced risk is related to the severity of visual impairment: totally blind women are least at risk and have about 50 percent of the risk of the sighted population. One potential explanation for this finding is that totally blind women have more melatonin, which protects them against cancer risk. So far, however, several small studies have not found their melatonin levels to be any higher than those of the normal population, although larger studies need to be performed to test this idea fully. There are also many other possible explanations for why totally blind women appear to be at a reduced risk, for example differences in diet, drug and alcohol use, or reproductive history.
But one possible cause of cancer development is the twenty-four-hour body clock--circadian system--which controls the timing of many biological processes, including the cell. Because they lack light perception, over half of totally blind people cannot maintain their body clocks on a twenty-four-hour day and, as a result, suffer from cyclic sleep disorders. The possible role of the circadian system in cancer development is an area of active research and may also be found in shift workers and flight attendants, who have problems remaining synchronized to the twenty-four-hour day.
In this month's issue of the Braille Monitor Brigham and Women's Hospital and Harvard Medical School in collaboration with the NFB are beginning a nationwide survey of health and sleep in visually impaired women aimed at addressing why blind women appear to have less risk of developing breast cancer. If the factor(s) reducing risk can be identified, this information can provide health and lifestyle advice for all women, not just visually impaired women, to reduce their risk of developing breast cancer.
If you are interested in assisting with this study, consult the miniature describing it in detail in the "In Brief" section.
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