Electric light makes for safer nights, but scientists are concerned that our artificially extended days may be harmful: they’ve officially classified night-shift work as "probably carcinogenic".
Light pollution?
“Light pollution” seems like a non-issue concocted by the lunatic Green fringe. The International Dark-sky Association (IDA), for example, uses terms like “light trespass”, without a glimmer of humour, for when the neighbour’s security light shines into your bedroom. Milky Way, they remind us, is not just a candy bar. The IDA’s mission is to save the night sky, a treasure lost to the glare of city lights.
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But there are reasons to dim the lights beyond the merely aesthetic. Firstly, our glowing cities contribute significantly to carbon emissions, and secondly, a growing body of evidence suggests that exposure to too much artificial light could have serious health consequences.
Of these, the most worrying is cancer: studies suggest that people who work unusual hours that expose them to many hours of artificial light, such as night nurses and airline staff, are at significantly increased risk for certain cancers, prompting the World Health Organisation to add night-shift work to the “probably carcinogenic” list, which includes other prime suspects like UV rays and diesel fumes.
Why we need the dark
It’s in pure, unpolluted darkness that our bodies produce melatonin, the "sleep hormone". Melatonin helps regulate the body’s intricate biological clock mechanisms, and has a major influence on a range of important biochemical processes.
Levels of the hormone are naturally lowest in the daylight hours, but increase in the evening. Photoreceptors in the eye register dimming light levels, and stimulate the pineal gland in the brain to increase melatonin production accordingly.
The concern is that too much light at night could inhibit melatonin production. We’re all familiar with the fact that disturbed sleep (caused, for example, by shining a bright light on the sleeper’s face) interferes with the body’s rhythms, and causes sluggishness, poor concentration, depression and irritability.
But recent studies provide disturbing evidence that reduced melatonin might be a causative factor in serious health problems in the long term.
Artificial light and cancer
Melatonin is thought to help protect genetic material from the mutations that lead to cancer. Thus artificially extended days, and the melatonin suppression this may cause, may increase the risk for certain cancers.
Breast cancer has been the most intensively studied as regards the effects of light exposure. Reduced melatonin production is linked to increased production of the hormone oestrogen by the ovaries. This, in turn, is associated with an increased risk for breast cancer.
Studies on women who work the night shift in artificial light have shown that their incidence of breast cancer increases, and it increases even more with longer shifts and the number of years they spend working at night.
Further, it appears that blind women are at significantly lower risk for breast cancer, which may be because they have consistently higher melatonin levels: A 2001 study by the Cancer Registry of Norway showed that totally blind women have a 36% lower risk of breast cancer compared to sighted women.
In the last couple of years, evidence has been mounting that the night-shift predisposes workers to endometrial, prostate and colorectal cancer as well.
Scientists have also speculated that artificial light could be one of the culprits responsible for the steady increase in the incidence of childhood leukemia – up by about 50% over the last fifty years in children under five.
Over the last half-century, children, particularly in the developed world, have been exposed to increased amounts of artifical light from various sources – TVs and computers as well as general night lighting.
Artificial light and precocious puberty
The incidence of “precocious” puberty – when signs of puberty appear earlier than normal – is increasing throughout the developed world, particularly among girls. One suspected factor is their average weight increase; another is reduced melatonin levels.
Melatonin normally acts to delay the onset of puberty. An Italian study found that watching too much television may upset children’s hormonal balance, reduce melatonin levels, and lead to early puberty. Children who took part in the study usually watched an average three hours’ TV a night. For one week, their parents denied them access to TV, computers and video games, and reduced their exposure to artificial light generally. The kids' melatonin levels shot up.
Too much screen time also encourages children to be sedentary, making them more likely to become overweight – which is thought to further suppress melatonin production. Apart from precocious puberty, decreased melatonin levels could disturb sleep patterns and cause hyperactivity.
Artificial light and short-sightedness
Concern about what the “artificial day” was doing to the developing eye prompted a 1999 study into the effects of those comforting night-lights in children’s bedrooms. The results showed that children who used night-lights up to age two had a significantly greater chance of developing short-sightedness.
Previous studies on chicks' eyes, which are a good comparative model for the human eye, showed developmental abnormalities if the chicks were exposed to high levels of artificial night light.
What’s to be done?
More research is needed before we have a clearer idea of the complex ways our bodies respond to light, and can separate its effects from those of other environmental factors.
Meanwhile, the cautionary message seems to be, as it so often is: refer back to nature.
Follow the dark-light sleep-wake cycle, as our ancestors did, by turning in early and rising with the sun.
If you do shift work, make sure that when you do sleep, you do so in a well-darkened room. Try to organise with your employer that you do fixed shifts rather than rotating shifts: the latter is thought to be far more disruptive to the sleep-wake cycle, and carries potentially higher health risks.
Use blinds or curtains to block “pollution” from street lights.
Rather don’t use night-lights for children, or wean them off these by gradually dimming the source or moving it away from the bedside. And it won’t hurt to cut down on the time kids (and others) spend watching TV or surfing the ‘Net.
Information sources
Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. Journal of the National Cancer Institute. October 2001. 17;93(20):1557-62.
International Dark-sky Association. www.darksky.org
Kliukiene J et al. Risk of breast cancer among Norwegian women with visual impairment. British Journal of Cancer (2001) 84:397-399
Kubo, T, et al. Prospective cohort study of the risk of prostate cancer among rotating-shift workers: findings from the Japan collaborative cohort study.American Journal of Epidemiology. 2006 Sep 15;164(6):549-55.
Quinn, G.E., M.G. Maguire et al. 1999. Myopia and ambient lighting at night. Nature. 399 (May 13):113.
Reuters. Night light linked to rise in child leukemia. 8 September 2004. MSNBC News.
Schernhammer ES, Schulmeister K. Melatonin and cancer risk: does light at night compromise physiologic cancer protection by lowering serum melatonin levels? British Journal of Cancer. 2004 Mar 8;90(5):941-3. Review.
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