Abstract: A branch of science believes bedtime reading increases the risk of breast cancer…
For a parent, there are few things more rewarding than the excitement shown by a child when reading them a bedtime story; but there’s a branch of science that fears that such a critical parenting role may increase a child’s risk of developing the most common cancer found in South African women.
Such a summation may not seem out of place in the unfortunately imbalanced rhetoric of poor health reporting typically found in tabloids. You can imagine the headline: “Mother Goose causes cancer!” But the reality is that those conducting research in the discipline of chronobiology – a relatively new branch of science concerned with the internal biological clocks of various living organisms – are concerned that using artificial light at night poses a risk of developing breast cancer.
A link between artificial light and breast cancer? It may sound far-fetched, but it’s something that has caught the eye of specialists in the field of cancer research. Anna-Mart Engelbrecht, Associate Professor at the Department of Physiological Sciences at Stellenbosch University says, “compelling evidence exists for an association between night work and breast cancer risk – a meta-analysis [published in Cancer Letters in August 2009] of eight studies suggested a 40% increased risk of breast cancer among women who work night shifts.”
Carol Benn, a specialised surgeon and breast cancer specialist, and co-founder of the Netcare Breast Care Centre of Excellence, confirms the connection, “the data is there”. She points to a number of epidemiological studies that have also confirmed an association between night-shift work and breast cancer. However, Dr Benn, suggests caution before leaping to conclusions, adding the nature of the connection is not so clear-cut, “it could be a classic ‘chicken or egg’ scenario.”
Dr Carl Albrecht, the Executive Manager of Research at the Cancer Association of South Africa (CANSA), agrees: “If you say “a link”, that implies a reasonably strong cause and effect relationship. This is not the case. So far the association between breast cancer and night shift work is only regarded as ‘probable'”. He is referring here to the finding of a working group established by the World Health Organisation’s International Agency for Research on Cancer (IARC) that shift work that disrupted the circadian rhythm – or internal 24-hour body clock – is “probably carcinogenic”.
“Probably”? Hermeneutics aside, the report admits its summation is based on “limited evidence in humans” and the few experiments have been conducted on animals and the “carcinogenicity of light during the daily dark period”.
The reality is that if there is some connection between artificial light and breast cancer, it’s actually not all that much to do with the light itself; it’s to do with its role as a trigger mechanism. The key is melatonin, a hormone secreted by the pineal gland in the brains of vertebrates, and which regulates certain cycles within the body.
In humans the primary function of the melatonin is the regulating of the daily sleep-wake cycle. As darkness descends at the end of the day, the reduction of light to the retina prompts the pineal gland to release melatonin, which in turns induces sleepiness.
It all sounds like a natural and simple process, but in today’s technologically advanced living environment, this ‘circadian rhythm’ is continually interrupted by omnipresent artificial light; and it is this impact that interests chronobiology.
However, although the link between artificial light and the disruption of production of melatonin is well documented, harder to prove is any causative link between such a disruption and breast cancer.
There is a connection, though. Dr Benn explains that when secreted at night, melatonin helps decrease the circulation of oestrogen in the body. This is important because breast cancers often “feed” on oestrogen. In a way, melatonin has similar properties to tamoxifen, a hormone therapy often used to treat breast cancer, which blocks oestrogen from reaching cancer cells. “Furthermore”, adds Dr Benn, “melatonin is an antioxidant that acts on tumour cells and results in cell death via various proposed mechanisms”.
This means that if levels of melatonin in the body were reduced in any way it might impede the suppression of cancer growth; it does not, however, suggest that a reduction in melatonin might cause cancer. It’s an important distinction.
Prof Richard G. Stevens insists, however, there’s more to the story. He is a cancer epidemiologist based at the Department of Community Medicine and Health Care at the University of Connecticut School of Medicine, and one of the driving forces behind chronobiology.
In the late 1970s Prof Stevens became perplexed with what he called “the confounding mystery” of why breast cancer risk rises so dramatically as societies industrialise. In 1987 he first proposed the theory that the use of electric lighting, resulting in lighted nights, might produce “circadian disruption” causing changes in the hormones relevant to breast cancer risk.
If Prof Stevens is correct then epidemiological studies of incidences of breast cancer should show higher incidences of breast cancer amongst women living in urban areas of extreme latitude, such as Reykjavik in Iceland and Dunedin in New Zealand, where winters are associated with long nights and longer exposure to artificial lighting. In South Africa, incidences of breast cancer should be higher in Cape Town than in, say, Musina. Yet, there is no evidence to support any of this.
Research conducted by IARC in Lyon in France, and the Department of Epidemiology and Public Health, Queen’s University Belfast in the UK, into changing global patterns of female breast cancer incidence shows that the highest incidence rates of breast cancer occur in northern and western Europe, northern America, Australia, New Zealand, and in the southern countries of South America.
On closer examination though there are notable variations within neighbouring countries of the same latitude, between different states or provinces in the same country, even between women of different race groups in the same city.
A possible answer lies in the socio-economic statuses of the countries in question. Most of the countries listed above are developed countries. Although that means a higher prevalence of artificial lighting, it also means different consumer behaviour in terms of diet and living standards, and thus exposure to different potential carcinogens. Importantly it also means differences in reproductive behaviour and access to oral contraceptives and hormone replacement therapy, known in cases to increase the risk of breast cancer.
A population-based study published in August in The Lancet seems to support this. It examined the global changing patterns of cancer according to the Human Development Index (HDI) – a composite indicator of life expectancy, education, and gross domestic product per head. It found that whereas cervical cancer (which is usually infection-related) was the most prevalent cancer amongst women in low HDI countries; breast cancer became more prevalent as countries became more developed.
So, if access to artificial lighting is an expression of a higher development status, and that status invites other lifestyle changes and shifts in consumptive behaviour, is it fair to isolate artificial lighting and its possible disruption of the body’s circadian rhythm and melatonin production as a cause of an increase in the risk of breast cancer?
Prof Stevens remains unshakeable in his conviction. He points to research in circadian disruption and prostrate cancer, and recently even stepped up the tone in an article submitted to the journal Cancer Epidemiology, Biomarkers and Prevention titled Does Electric Light Stimulate Cancer Growth in Children? He argues that not only could the risk of developing cancer increase in children exposed to artificial light, but that “maternal exposure to electric light during the night might also cause changes in foetal development”.
The implications of Prof Stevens’ assertions are frightening, because it means that if the simple act of switching on a light at night suppresses the natural secretion of melatonin and sets in motion a physiological shift that increases a risk of developing cancer, then a father’s reading a bedtime story to his child, or a mother night feeding a newborn baby, may be more harmful than good.
When pressed for proof of such malignant cause and effect between artificial lighting and breast cancer, Prof Stevens is both adamant and diplomatically circumspect, and invokes a legal analogy: “It’s guilty in a civil trial, but no verdict in a criminal trial. A reasonable jury would say there is a preponderance of evidence, but it’s not beyond a reasonable doubt at this point.”
Originally published in Mail&Guardian 9th November 2012.