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The dark side of light

MagazineJune 2009 (Vol. 20 Issue 3)The dark side of light

Without question, the light bulb is one of the greatest inventions of all time, making human activity possible beyond daylight hours

Without question, the light bulb is one of the greatest inventions of all time, making human activity possible beyond daylight hours.
However, as an example of 'too much of a good thing', the modern world is now so awash with artificial light that, in many places, true darkness has all but disappeared. This has been a well-known problem for flora and fauna, but the effects
of light pollution on humans are only just beginning to surface. Now, provocative new evidence suggests that too much light at night could be increasing the risk of cancer.
In a study just published in the journal Chronobiology International, Israeli researchers analyzed cancer rates and satellite images from 164 countries, and found an indisputable correlation between prostate cancer and levels of nighttime artificial illumination.
Countries where nighttime artificial lighting was used most intensively had rates of prostate cancer that were 110 per cent higher than in countries with the lowest levels of artificial light at night (Chronobiol Int, 2009; 26: 108-25).
Although these findings do not constitute proof of a cause-and-effect relationship, they do add to the results of an earlier study by the same team in which they uncovered an intriguing link between light pollution and breast cancer.
Using a similar study design, the researchers looked at 147 commun-ities in Israel and found that women living in neighbourhoods where it was bright enough to read a book outside at midnight had a 73-per-cent higher risk of developing breast cancer compared with those living in areas with the least outdoor artificial lighting. This increased risk was apparent even after controlling for other factors that might have had an influence on cancer risk, such as ethnicity, affluence and air pollution (Chronobiol Int, 2008; 25: 65-81).
The two studies taken together suggest that light pollution may have serious health effects for both men and women-particularly in places where there is a lot of artificial light at night, such as London, Manhattan and Las Vegas. But how can nighttime artificial lighting possibly cause cancer?
The melatonin connection
The leading theory is that exposure to too much light at night disrupts the body's production of melatonin, a hormone known to help regulate our internal biological clock. Melatonin is produced by the brain's pineal gland in the absence of light, but its release is suppressed as soon as light hits the eyes. This means that light invading our bedrooms from outside can cause melatonin levels to plum-met. Depressed melatonin levels have, in turn, been associated with increased rates of cancer.
Night-shift workers, for instance-a population that is intentionally exposed to light at night on a regular basis-are known to exhibit lower levels of nighttime melatonin. What's more, studies consistently show an increased risk of breast cancer in these individuals and, more recently, endometrial cancer, too (Cancer Lett, 2008 Dec 11; Epub ahead of print). In one of these studies, 'graveyard' shiftwork was associated with a 60-per-cent higher risk of breast cancer, with a trend towards increased risk with the increasing years and more hours of shiftwork per week. Interestingly, an increased breast cancer risk was also suggested for those shift workers who slept in the brightest bedrooms (J Natl Cancer Inst, 2001; 93: 1557-62).
The findings of yet another US study provide what the authors call
"a rational biological explanation for the increased breast cancer risk in female night-shift workers". Using laboratory rats with tumours made up of human breast cancer cells, the scientists evaluated how nighttime fluorescent light affects tumour growth by injecting the animals with blood samples from healthy women. These samples contained varying concentrations of melatonin, depending on the time of day they were collected (during daylight hours, during the night after two hours of complete darkness and during the night after exposure to 90 minutes of artificial light). The results showed that tumour growth increased on exposure to melatonin-deficient blood-in other words, the blood collected during the daytime or after exposure to light at night. On the other hand, the blood samples that were rich in melatonin-in other words, those collected during complete darkness-were found to slow tumour growth significantly (Cancer Res, 2005; 65: 11174-84).
Besides breast cancer, night-shift work has also been linked to colo-rectal and endometrial cancer in women (J Natl Cancer Inst, 2003; 95: 825-8; Cancer Res, 2007; 67: 10618-22), and prostate cancer in men (Scand J Work Environ Health, 2008; 34: 5-22). Although not conclusive, the research linking night-shift work and cancer was convincing enough for the Inter-national Agency for Research on Cancer (IARC), in 2007, to classify this type of job as a probable human carcinogen (J Pineal Res, 2009 Feb 9; Epub ahead of print).
Nevertheless, as the latest studies have found, it's not just shift workers who are at risk. The recent research linking light pollution to increased rates of breast and prostate cancer in the general population suggest that suppression of melatonin by artificial light at night might be a key contrib-utor to the overall incidence of cancer. Clearly, more research is needed although, so far, it appears that getting enough darkness is an important factor to prevent cancer.

Other health effects
Apart from cancer, too much nighttime light can also lead to a range of other health effects-from cardiovascular disorders to vision problems. As one 'mini-review' explains, "Interruptions in normal circadian light cycles and the resulting disruption of normal melatonin rhythms cause widespread disruptive effects involving multiple body systems, the results of which can have serious medical conse-quences for individuals" (J Pineal Res, 2007; 43: 215-24).
Again, shift-work studies form the basis of the evidence in this review, which showed increased rates of heart disease, digestive problems, sleep disorders and poor pregnancy outcomes in such employees (Cardio-vasc J Afr, 2008; 19: 210-5; Environ Health Perspect, 2009; 117: A20-7).
In one especially alarming study, pregnant women who regularly worked the night shift were 85-per-cent more likely than daytime workers to suffer a miscarriage late in pregnancy or to have a stillbirth
(J Occup Environ Med, 2004; 46: 1144-9). The same Danish research team also found a connection between shift work and low birth weights (Am J Obstet Gynecol, 2004; 191: 285-91).
Equally worrying is the research linking shift work to cardiovascular effects. According to a recent review of the literature so far, hypertension, left ventricular hypertrophy (abnor-mal thickening of the lower left chamber of the heart), coronary heart disease and myocardial infarction (heart attack) all occur more frequently, and tend to be more severe, in night-shift workers than in daytime workers (Cardiovasc J Afr, 2008; 19: 210-5).
There are a number of possible explanations for this, but the evidence to support the role of melatonin appears to be particularly convincing (J Pineal Res, 2007; 43: 215-24). Indeed, one (albeit small-scale) study found that patients with coronary heart disease had significantly lower melatonin levels at night compared with patients without heart disease (Lancet, 1995; 345: 1408).
In addition to the shift-work studies, there is also evidence to suggest that too much light at night in our homes may have detrimental health effects, too.
A 1999 study by a team of researchers at the University of Pennsylvania Medical Center and Children's Hospital of Philadelphia reported that night-time light exposure during the first two years of life may contribute to the develop-ment of myopia, or short-sightedness (Nature, 1999; 399: 113-4).
As reported by the BBC News at the time, children under the age of two who slept with the lights on were up to five times more likely to be short-sighted later on than infants who slept in the dark, while toddlers who slept with a night-light were three times more likely to be myopic (http://news.bbc.co.uk/1/hi/health/342256.stm).
Although other studies have failed to replicate the above results (Br J Ophthalmol, 2003; 87: 580-2), a study of school children in Singapore and China did find a weak, indirect association between night-lights and myopia (Arch Ophthalmol, 2002; 120: 620-7). Also, another US study from the Philadelphia team found a significant association between the progression of myopia and the number of hours of complete darkness to which young adults were exposed at night (Ophthalmology, 2002; 109: 1032-8).
Beyond short-sightedness, there is also evidence that excessive light exposure early in life might be contributing to an increased risk of depression and other mood dis-orders. The research is still in its early stages but, according to Vanderbilt University researcher Douglas McMahon, "the data would indicate that human infants benefit from the synchronizing effect of a normal light/dark cycle" (Environ Health Perspect, 2009; 117: A20-7).

Still in the dark
So, it seems that exposure to too much light at night-whether from obtrusive and unnecessary outdoor lighting, children's night-lights or long-term night-shift work-can disrupt our natural bodily rhythms with wide-ranging effects. Although more research is needed to clarify such associations, particularly in terms of the health effects of light pollution, it appears to be already evident that darkness-just like sunlight-is vital for health.
Joanna Evans

Forms of light pollution

- Sky glow is the bright halo that appears over urban areas at night, the result of light being scattered by water droplets or particles in the air
- Light trespass occurs when unwanted artificial light from, say, a streetlight spills onto the adjacent property, lighting an area that would otherwise be in the dark
- Glare is created by light that shines horizontally
- Overillumination refers to the use of artificial light well beyond what is needed for a specific activity, such as keeping the lights on in an
empty office building throughout the night (Environ Health Perspect, 2009; 117: A20-7).

Getting a dark night's sleep

- Avoid bright lights within three hours of bedtime
- Keep computer time at night to a minimum
- Use blackout blinds or thick curtains in bedrooms to block out the glare from streetlights
- Try not to get up in the middle of the night. If you do, minimize your exposure to bright light by, for example, installing a dim red bulb in the bathroom as a night light.

Who's thinking?

Decision-making may be an aspect of rational thought, but new research suggests that our feelings play a vital role in the process

How do we make decisions? Most of us assume it's an analytical process in which we carefully-or sometimes speedily-weigh the pros and cons. However, new research suggests that decision-making is a complex mix that has far more to do with our feelings than rational judgement.
A major proponent of the theory is neurologist Antonio Damasio, whose speculations started after he read about people who, because of injuries following an accident or surgery, could no longer use the feeling part of their brain in making decisions.
One such case was railway worker Phineas P. Gage who, in 1848, lost the use of the frontal lobe of his brain after an explosion sent an iron bar through his head. Although Gage continued to function normally, his physician noted that Gage became socially inappropri-ate and highly profane. He also became incapable of making long-term plans.
His was a case that fascinated Damasio, who noted that Gage's mental skills for language, memory and perception were all intact, but his ability to make value judgements had been destroyed by the accident.
Recreating Gage's injuries with a model of the brain, Damasio found that the iron bar had sliced through the ventromedial (or central underbelly) region of the frontal lobe. From this, Damasio suggests that reasoning and emotion are carried out by different parts of the brain, and both are essential for making good decisions.
He also quotes a more recent case of a man in his 30s called Elliot, who became detached and imperturbable following surgery to remove a brain tumour. Like Gage, he was unable to make decisions, didn't learn from experience and eventually lost his job-and his wife left him.
Damasio gives two examples to demonstrate how Elliot's personality changed. In the first, he drove through very hazardous icy conditions, avoiding every slippery patch and overturned car, without even breaking out in a sweat. The following day, Damasio asked him to choose a date for his next appointment, and Elliot suffered a severe panic attack until Damasio put him out of his misery and selected the date for him.
Most of us are the complete reverse of Elliot and Gage, believes neuro-scientist Jonah Lehrer, and our emotions are making the decisions
for us. More precisely, he believes that dopamine, a neurotransmitter that he describes as "the molecule of intui-tion", often forces us to do some-thing-such as place another bet or buy another handbag-that common sense would consider to be against our own best interests.
Lehrer cites one experiment in which two groups of students were given either a long or short number to remember. They were then told to move to another room and, on the way, they were offered either fruit or chocolate cake. Almost all of those
who had to remember a long number took the cake, while the short-number group chose the fruit. Why? Because those in the long-number group were more preoccupied and, so, took a decision against their own best interests.
Conversely, other experiments have suggested that we make our best decisions when we are distracted. A Dutch experiment found that people were more likely to choose the most suitable car if they stopped thinking about it, distracted themselves and then made their decision based on the first thing that came into their head.
Lehrer argues that dopamine has its own rationality, often based on previous success, and that is why the distracted car buyers made the right choice without 'thinking' about it. That would also explain why sportsmen often make the right decision, usually under intense pressure, without consciously thinking about their next move. When athletes do think about what they're doing, their game can fall apart, as happened with golfer Jean Van de Velde, who started to think about his stroke play at the final hole-and threw away a three-shot lead at the 1999 Open championship.
Distracted car buyers and athletes suggest that we often make the best decisions when we're not thinking about it. But, sometimes, the rational mind has to take precedence-for instance, when working out a problem.
This was illustrated by an experi-ment by psychologist Carol Dwecke, who told one group of schoolchildren after a test that they were clever, whereas the other group was congrat-ulated for working hard. In a second test, the 'clever' group's scores dropped by around 20 per cent, while the 'hard workers' rose by 30 per cent.
Evidently, the hard-workers self-corrected and thought about what they were doing, while the clever ones believed that all they had to do was turn up for the test.
Educationalist Ken Robinson isn't sure that the unthinking decisions are made by dopamine; instead, he believes that intuition comes from the enteric nervous system, which he describes as a second brain within the intestines.
But all agree that we know little about the workings of the human brain, about feelings and how deci-sions are made. Robinson quotes psychologist William James, who said: "The greatest discovery of my genera-tion is that human beings can alter their lives by altering their attitude of mind. If you change your mind, you can change your life."
True, but a hundred years on, we are still no nearer to understanding how.
Bryan Hubbard



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