New Danish Nurses’ Breast Cancer/ Shift Work Studies (1) Miss a Much Bigger Problem According to Dr. Hanslet, Author of "Heroes of Cancer Prevention Research" (Amazon)

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"Using light at night when melatonin would otherwise be produced, increases cancer risk for everyone" says Hansler, veteran of 42 years with GE Lighting. According to him, the use of nurses who work day shift as controls in the study is questionable since day shift nurses also are exposed to a lot of light at night and experience loss of melatonin (2). This partially masks the damaging effect of night-long exposure. Fortunately, it is primarily the blue rays in ordinary white light that cause the problem. Light bulbs that don’t produce blue rays and glasses that block blue light are available from Photonic Developments at a company founded by Hansler in 2005.

The latest nurses’ study once again focuses on the increased cancer risk for nurses who work night shifts compared to nurses who only work days. Using nurses who only work days as controls in the study answers the particular goal of the study, namely, is night shift work an occupational hazard compared to day shift. It is misleading, according to Hansler, in that is doesn’t acknowledge that the incidence of breast cancer among day working nurses is already elevated along with that of the general population. It is elevated compared to women who live in countries without electric lighting according to World Health Organization statistics. It is also elevated compared to women who are totally blind (about half that of women with normal vision) (3).

When the eyes are exposed to light in the hours before bedtime, it prevents the pineal gland from producing melatonin, a hormone that fights against cancer. Before the arrival of electric lights humans had an average of 12 hours of darkness a night. In darkness, humans produce melatonin an average of 11.4 hours a night (4). Most Americans (and Danes as well) are only in darkness for 7 or 8 hours a night and produce melatonin for a shorter time. It is this decrease in the time when melatonin is present in the blood that is believed to contribute to the increase in the incidence of cancer (5). Extensive animal studies done some twenty years ago showed very clearly that reducing the time when melatonin was present in the blood, due to extended exposure to light, increased the incidence of breast cancer in the animals.

More recent studies (6) use human breast cancers grown on animals, but supplied with human blood, to note the effect of melatonin. Cancers grow slowly or not at all when the blood contains melatonin. They grow rapidly when the blood does not contain melatonin. These studies helped convince a committee of the World Health Organization to classify light exposure that causes circadian disruption as a probable carcinogen.

Because no expensive pills are involved in a switch to low blue light bulbs, funding for clinical trials of their use must rely on the government. Use of glasses that block blue light must also await clinical trials. In the meantime, reading Dr. Hansler’s new book “Heroes of cancer prevention research” (Amazon) may convince some people that there is enough evidence already available for them to make a very simple, inexpensive change in life style. The book is available at

(1) Eur J Cancer. 2012 Jul;48(11):1722-9. Epub 2011 Aug 16.
Case-control study of shift-work and breast cancer risk in Danish nurses: impact of shift systems.
Hansen J, Stevens RG.

(2) J Clin Endocrinol Metab. 2011 Mar;96(3):E463-72. Epub 2010 Dec 30.
Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans.
Gooley JJ, Chamberlain K, Smith KA, Khalsa SB, Rajaratnam SM, Van Reen E, Zeitzer JM, Czeisler CA,Lockley SW.

(3) Cancer Causes Control. 2009 Nov;20(9):1753-6. Epub 2009 Aug 1.
Total visual blindness is protective against breast cancer.
Flynn-Evans EE, Stevens RG, Tabandeh H, Schernhammer ES, Lockley SW.

(4) PLoS One. 2008 Aug 26;3(8):e3055.
Individual differences in the amount and timing of salivary melatonin secretion.
Burgess HJ, Fogg LF.

(5) Mutat Res. 2009 Nov-Dec;680(1-2):106-8.
Working against our endogenous circadian clock: Breast cancer and electric lighting in the modern world.
Stevens RG.

(6) Cancer Res. 2005 Dec 1;65(23):11174-84.
Melatonin-depleted blood from premenopausal women exposed to light at night stimulates growth of human breast cancerxenografts in nude rats.
Blask DE, Brainard GC, Dauchy RT, Hanifin JP, Davidson LK, Krause JA, Sauer LA, Rivera-Bermudez MA,Dubocovich ML, Jasser SA, Lynch DT, Rollag MD, Zalatan F.

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Richard Hansler
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