Toronto Globe and Mail
January 14, 2006


Nighttime exposure to electric lighting curbs production of vital
melatonin, researchers find

By Martin Mittelstaedt

The high rate of breast cancer in industrialized countries has long
puzzled medical researchers, but a team of U.S. scientists has
discovered a possible explanation for why women in developed countries
are at high risk of developing the disease.

The answer at first glance may seem unlikely: nighttime exposure to
electric lighting.

In a major breakthrough, researchers have linked exposure to light at
night to the growth in breast-cancer tumours. The tumours grew because
artificial light interfered with the ability of women to create
melatonin, the hormone that regulates the body's daytime and night

The discovery holds major public-health implications because most
women in industrial societies turn on lights at night in their homes
and offices and may potentially be at risk from this exposure.

"Light, in terms of our experiments, stimulates breast-cancer growth
activity, and obviously this is due to the ability of light to shut
off melatonin production," said David Blask, a scientist with the
Bassett Research Institute in Cooperstown, N.Y., who led the team that
made the discovery.

He said, "melatonin puts cancer cells, in particular breast-cancer
cells, to sleep at night," but if the levels of this hormone are
diminished by exposure to light at night, cancers "become insomniacs"
and grow all the time.

In recent years, there has been a flurry of research suggesting light
at night may be a health hazard, causing illnesses ranging from
chronic fatigue to depression.

But until now, there has only been circumstantial evidence linking it
to breast cancer. For instance, women who regularly work overnight
"graveyard" shifts have been found to have an elevated incidence of
the disease, in some cases up to 60 per cent higher than those who
work regular day shifts.

This new research, outlined last month in the journal Cancer Research,
is the first experimental evidence to show that light at night can
have an effect similar to a cancer-promoting chemical.

"Electric lighting as a driver of the breast-cancer epidemic worldwide
-- that's a dramatic big thing, and new," said Richard Stevens, an
epidemiologist at the University of Connecticut who has studied the
health risks of light pollution.

The U.S. National Institutes of Health, which financed the study,
hailed the results and said they offer "a promising new explanation
for the epidemic rise in breast-cancer incidence in industrialized
countries like the United States."

Les Reinlib, a program administrator for the agency, said the
discovery may hold promising avenues for preventing breast cancer with
simple steps, such as changing women's exposure to light at night. He
also said that melatonin, an inexpensive and widely available hormone
supplement, should be studied to see whether it holds promise as an
anti-cancer therapy.

Breast cancer is the most frequently diagnosed cancer among Canadian
women, with about 21,600 new cases diagnosed annually. About 5,300
women die each year of the disease.

Dr. Stevens said the high breast-cancer incidence in industrialized
countries, at about five times that of poor countries, has long
intrigued researchers. "We have what I would call an epidemic of
breast cancer and we don't know why," he said.

Studies into possible explanations, such as the high-fat Western diet,
pesticides, or industrial pollutants, have generally been
inconclusive, suggesting that there is something else that is
ubiquitous in affluent countries that is causing the disease.

Breast cancer is linked to genetics, early menstruation, and
reproductive history, among other things, but about 60 per cent of
those with the disease have no currently known risk factor.

The researchers found that melatonin plays a key role in inhibiting
the growth of breast-cancer tumours. Melatonin is produced in the
brain's pineal gland, guided by cues from the retinas in the eyes, and
circulates in the blood. The hormone is produced only when it is dark,
beginning at nightfall, with production peaking in the middle of the
night, and then shutting off during the day. When people are exposed
to light at night, the body thinks it is daytime and melatonin
formation stops.

To track melatonin's impact, Dr. Blask's team implanted human breast-
cancer tumours in rats, then isolated the tumours so they were fed
blood from a single artery and drained by only one vein. They then
pumped blood from premenopausal female volunteers through the cancer

Melatonin-rich blood drawn from women who were in darkness markedly
suppressed the growth of the tumours. But when the women were exposed
to fluorescent light at night, causing their melatonin levels to drop,
tumour growth took off. Tumour growth also increased when the breast-
cancer cells were exposed to melatonin-deficient blood collected
during daytime.

The melatonin-rich blood was drawn from women at 2 a.m., after two
hours of complete darkness in an office room. They were exposed to 90
minutes of bright, fluorescent office-style lighting, after which
another blood sample, this one with low melatonin, was taken. The
daytime blood sample was taken from 10 a.m. to 2 p.m., a period when
melatonin levels normally fall because of exposure to sunlight.

The experiment represented as close to a human test as possible
without actually using women as test subjects, something that would be
impractical and unethical. But because the tests were conducted using
human blood and human cancer cells, the scientists feel certain the
experiment indicates what is happening in women.

Although the research is considered a breakthrough, it has
limitations. For one thing, it didn't indicate what levels of lighting
are safe. The bulb used in the experiment was an ordinary fluorescent
office light, but the brightness of the light was high -- about the
amount used at a drafting table.

Researchers have found that almost all electric lighting has some
melatonin-dampening effect, but they have yet to work out a dose-
response relationship for cancer cells.

Nor did the study address what initially causes women to develop
cancer cells. Scientists believe almost everyone generates some cancer
cells during their lives, but the immune system manages to contain the
aberrant cells before they develop into dangerous tumours.

In a separate test, the U.S. researchers found that melatonin blocks
the growth of liver tumours in rats, suggesting a far wider impact
than on breast cancer alone. Many researchers suspect that the
incidence of prostate cancer, a disease in men in industrialized
countries that has had the same explosive growth as breast cancer
among women, might also be linked to light pollution.

Dr. Blask said melatonin has a beneficial effect because it literally
starves the cancer of material needed to grow. It blocks the
absorption into cancer cells of a compound contained in
polyunsaturated dietary fat, from foods such as corn oil, that spurs
tumour growth.

He speculated that light at night causes a melatonin deficiency,
allowing breast-cancer cells to proliferate fast enough to gain the
upper hand over the body's defences.

"Melatonin is a layer of protection that you have at night," Dr. Blask

"When you have enough light present to suppress it, you lose -- you
eliminate -- that layer of protection."

What you can doResearchers who study the health effects of artificial
light at night say there is one commonsense way to minimize exposure:
Spend a reasonable amount of time at night in continuous darkness.

This allows your body to generate high levels of melatonin, the
hormone secreted by humans at night that limits the growth of breast
cancer and may help slow other tumours.

Russell Reiter, a professor of neuroendocrinology at the University of
Texas in San Antonio and an authority on melatonin, said almost
everyone in industrialized countries lives too much of their time at
night under electric light bulbs and is consequently melatonin-
deprived, in contrast to people living in a more natural environment.

"It's alarming in the sense that this may be a major ...
[contributing] factor for the incidence of breast cancer and maybe
other cancers," he said.

Dr. Reiter recommends that after you go to bed, avoid turning on
lights if you wake up. If you go to the washroom at night and turn on
a light, this tricks your body into thinking it is daytime, causing
melatonin production to cease. "One second of light in the middle of
the night is too much," he said.

He said the general glow of lights over cities at night is probably
not a health hazard. But as an additional safety precaution, some
researchers recommend that you make sure your bedroom is as dark as

Melatonin is produced by the brain's pineal gland when you are in
darkness at night, and has nothing to do with how long you sleep. The
crucial factor is to remain in darkness for as long as possible,
whether you are asleep or awake, according to Richard Stevens, an
epidemiologist at the University of Connecticut.

Dr. Stevens has conducted research showing that the amount of time a
woman spends in darkness causes a change in breast-cancer risk. Those
in darkness for nine or more hours a night have a lower risk than
those in darkness for only seven or eight hours.

It is only the blue part of the light spectrum that stops people from
producing melatonin. Dr. Reiter said light bulbs could be redesigned
to eliminate the frequency causing blue light, allowing lights to be
used at night without harming melatonin production.

Melatonin is available in supplements, but experts are divided on its
use as an anti-cancer therapy. Dr. Reiter predicts it will become more
widely used for this purpose.

But Dr. Stevens recommends that because the human body produces
melatonin, you should do things to maximize the amount your body
makes. "If melatonin is important, live a melatonin-friendly

Copyright 2006 Bell Globemedia Publishing Inc.