The idea that over-exposure to artificial light at night may disrupt the human biological clock and increase the risk of some cancers has received new support.
In February 2002, researchers announced the surprising discovery of a new photoreceptor system in the eye that helps set our body clocks. Now, neurologist George Brainard and colleagues at Thomas Jefferson University, Pennsylvania, have shown that this clock setting is determined by light of short wavelength, which is more prevalent in the artificial lighting used at night.
Epidemiological studies have shown that women who work night shifts are more likely to develop breast cancer, and that blind women have lower risks. Brainard says prostate cancer risk may also be increased by excessive light at night.
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He told the American Association for the Advancement of Science鈥檚 annual conference in Denver, Colorado, that the evidence was now 鈥渓ining up鈥� to back the theory that too much artificial light at night increased the risk of cancers.
24/7 society
鈥淎s we move towards a 24/7 society where we are constantly up,鈥� Brainard says, 鈥渨e have to wonder, are we introducing risk factors which could be better controlled?鈥� He recommends 鈥減rudent avoidance鈥� of light at night.
Cancer researchers agree that further work should be carried out to determine whether any link exists, but caution that the excess night light is likely to account for only a small minority of cancer cases.
Brainard鈥檚 team showed that the production of melatonin, one of the hormones that sets the human biological clock, is dictated by the amount of short wavelength light received by the eye.
These wavelengths are at the blue, indigo and violet end of the visible spectrum, colours more useful to detect at night. But they are also stronger in artificial light. This could depress melatonin production, which some studies show can slow the growth of breast cancer.