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01/12/2005 - Breast Cancer linked to "light at night"


In women the risk of developing breast cancer is up to five times higher in industrialised nations than in underdeveloped countries. In the UK, the incidence is increasing at around 3% per year. Overall, nearly 50% of breast cancers cannot be accounted for by conventional risk factors. In 2003, a report prepared for the Health & Safety Executive highlighted the increased breast cancer in night-shift workers (Ref 1).

Since the late 1980's, it has been suspected that people exposed to excessive "light at night", such as night-shift workers, may be at a greater risk of developing cancer, with breast cancer particularly implicated. A new study has elegantly demonstrated that melatonin, a natural hormone produced by the pineal gland at night time has a dramatic effect on cancer growth - a hormone that is not produced if the person is exposed to light through the night. EMF exposure has also been shown to reduce nocturnal melatonin.

Experiment summary

Breast cancer tumours were transplanted into rats with no immune system (known as "nude" rats) for the experiment. Young women volunteers were allowed to sleep in total darkness. In the early hours blood samples were taken automatically in a manner which did not disturb the women (which would contain the normal amount of nocturnal melatonin). This blood was injected into the rats, and the tumours stopped growing. Blood samples were then taken from the same women after they had been exposed to bright light which suppressed the production of melatonin. When this blood was given to the rats the breast cancers continued to grow. Finally, this second blood sample was supplemented with melatonin at a level equivalent to those in the first blood samples. When given to the rats, this again prevented the growth of the human breast cancers.

Other implications

These findings are potentially highly significant, especially when placed in the context of the increased incidence of leukaemias and other cancers in the last 50 years. Magnetic fields from powerlines and electrical appliances have been shown to suppress melatonin production in the general population, and it could be this secondary effect that is causing the association between powerlines and cancer. Also implicated is the large increase in cancers that started to increase at the same time as the electrification of houses in Western society.


1) "Melatonin-Depleted Blood from Premenopausal Women Exposed to Light at Night Stimulates Growth of Human Breast Cancer Xenographs in Nude Rats" - David E Blask, George C Brainard et al, Cancer Research, vol. 65 (23), 1-11

Available from: the American Association for Cancer Research - Cancer Research.

Paper abstract

The increased breast cancer risk in female night shift workers has been postulated to result from the suppression of pineal melatonin production by exposure to light at night. Exposure of rats bearing rat hepatomas or human breast cancer xenografts to increasing intensities of white fluorescent light during each 12-hour dark phase (0-345 pW/cm2) resulted in a dose-dependent suppression of nocturnal melatonin blood levels and a stimulation of tumor growth and linoleic acid uptake/metabolism to the mitogenic molecule 13-hydroxyoctadecadienoic acid. Venous blood samples were collected from healthy, premenopausal female volunteers during either the daytime, nighttime, or nighttime following 90 minutes of ocular bright, white fluorescent light exposure at 580 pW/cm2 (i.e., 2,800 lx). Compared with tumors perfused with daytime-collected melatonin-deficient blood, human breast cancer xenografts and rat hepatomas perfused in situ, with nocturnal, physiologically melatonin-rich blood collected during the night, exhibited markedly suppressed proliferative activity and linoleic acid uptake/metabolism. Tumors perfused with melatonin-deficient blood collected following ocular exposure to light at night exhibited the daytime pattern of high tumor proliferative activity. These results are the first to show that the tumor growth response to exposure to light during darkness is intensity dependent and that the human nocturnal, circadian melatonin signal not only inhibits human breast cancer growth but that this effect is extinguished by short-term ocular exposure to bright, white light at night. These mechanistic studies are the first to provide a rational biological explanation for the increased breast cancer risk in female night shift workers. (Cancer Res 2005; 65(23): 1-11)