Some researchers fear it could be linked to an increase in breast cancer in developing nations, writes Daryl Ilbury.
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 this parenting role may increase a child's risk of developing breast cancer, the most common cancer found in South African women.
News like this is usually associated with 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 living organisms, are concerned that using artificial light at night poses a risk of developing 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, an associate professor in the department of physiological sciences at Stellenbosch University, said: "Compelling evidence exists for an association between night work and breast cancer risk – a meta-analysis [published in the journal 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, confirmed the connection – "the data is there".
She referred to a number of epidemiological studies that had also confirmed an association between night-shift work and breast cancer.
But Benn advised caution and added that the nature of the connection was not so clear cut. "It could be a classic chicken-or-egg scenario."
Dr Carl Albrecht, executive manager of research at the Cancer Association of South Africa, agreed: "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 was referring to the finding of a working group established by the World Health Organisation's International Agency for Research on Cancer, which said shift work that disrupted the circadian rhythm, or internal 24-hour body clock, was "probably carcinogenic".
The report admitted that its summation was based on "limited evidence in humans" and the few experiments had 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 actually does not have all that much to do with the light itself; it has 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 that regulates some cycles in the body.
In humans, the primary function of 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.
Advanced living environment
It sounds like a natural and simple process, but in today's technologically advanced living environment the circadian rhythm is continually interrupted by omnipresent artificial light and it is the impact of this that interests chronobiology.
But although the link between artificial light and the disruption of production of melatonin is well documented, it is harder to prove a causative link between the disruption and breast cancer.
There is a connection, though. Benn said when melatonin was secreted at night, it helped to 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", Benn said, "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, but it does not suggest that a reduction in melatonin might cause cancer. It's an important distinction.
But there's more to the story, according to Professor Richard G Stevens, a cancer epidemiologist based at the department of community medicine and healthcare at the University of Connecticut school of medicine and a leading authority on chronobiology.
In the late 1970s he became concerned about "the confounding mystery" of why breast cancer risk rose so dramatically as societies industrialised. In 1987, he first proposed the theory that the use of electric lighting, resulting in lighted nights, might produce "circadian disruption" and cause changes in the hormones relevant to breast cancer risk.
If Stevens is correct, then epidemiological studies of incidences of breast cancer should show higher incidences of breast cancer among 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 this.
The International Agency for Research on Cancer in Lyon, France, and the department of epidemiology and public health at Queen's University in Belfast in the United Kingdom investigated the changing global patterns of female breast cancer and found that the highest incidence of the disease is in northern and western Europe, North America, Australia, New Zealand and the southern countries of South America.
But there were notable variations in neighbouring countries of the same latitude, between different states or provinces in the same country and even between women of different race groups in the same city.
A possible answer lies in the socioeconomic status of the countries. Most of those 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 some 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 changing global patterns of cancer according to the human development index, a composite indicator of life expectancy, education and gross domestic product per head. It found that cervical cancer, which is usually infection-related, was the most prevalent cancer among women in low-index countries, but breast cancer became more prevalent as countries 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 consumption, 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?
Stevens believes so. He referred to research in circadian disruption and prostrate cancer and recently even upped the ante in an article submitted to the journal Cancer Epidemiology, Biomarkers and Prevention titled "Does electric light stimulate cancer growth in children?" He argued that not only could the risk of developing cancer increase in children exposed to artificial light, but also that "maternal exposure to electric light during the night might also cause changes in foetal development".
The implications of Stevens's 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, a father reading a bedtime story to his child or a mother night feeding a newborn baby may be more harmful than good.
Pressed for proof of such malignant cause and effect between artificial lighting and breast cancer, Stevens was both adamant and diplomatically circumspect, invoking 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."
Daryl Ilbury, a former award-winning broadcaster, is a science journalist and columnist. He recently completed a master's degree in science journalism at
City University London