Results from a new study published in the December 1, 2005 issue of the
scientific journal Cancer Research show that in laboratory mice nighttime
exposure to artificial light stimulated the
growth of human
breast tumors by
suppressing the levels of a key hormone called
melatonin. The study also
showed that extended periods of nighttime darkness greatly slowed the
growth of these tumors.
The risk of developing breast cancer is about five times higher in
industrialized nations than it is in underdeveloped countries,These
Results suggest that the increasing nighttime use of electric
lighting, both at home
and in the workplace, may be a significant factor.
The study results might explain why female night shift workers have a higher
rate of breast cancer. It also offers a promising new explanation for the
epidemic rise in breast cancer incidence in industrialized countries like the
United States.
Previous research had shown that artificial light suppresses the brain’s
production of melatonin, a hormone that helps to regulate a person’s
sleeping and waking cycles. The new study shows that melatonin also plays a
key role in the development of cancerous tumors.
It is known that many tumors are largely dependent on an essential fatty
acid,
nutrient called linoleic acid, in order to grow. Melatonin interferes with the
tumor’s ability to use linoleic acid as a growth signal, leading to shut down
of tumor metabolism and growth.
According to the researchers, melatonin exerts a strong influence on the
body’s circadian rhythm, an internal biological clock that regulates sleep —
wake cycle, body temperature, endocrine functions, and a number of disease
processes including heart attack, stroke and asthma. “Evidence is emerging
that disruption of one’s circadian clock is associated with cancer in humans,
and that interference with internal timekeeping can tip the balance in favor
of tumor development,” said Blask.
“The effects we are seeing are of greatest concern to people who routinely
stay in a lighted environment during times when they would prefer to be
sleeping,” said Mark Rollag, Ph.D., a visiting research scientist at the
University of Virginia and one of the study co-authors. “This is because
melatonin concentrations are not elevated during a person’s normal waking
hours.”
“If the link between light exposure and cancer risk can be confirmed, it could
have an immediate impact on the production and use of artificial lighting in
this country,” said Richard Stevens, Ph.D., an epidemiologist with the
University of Connecticut Health Center who has authored several papers on
the subject. “This might include lighting with a wavelength and intensity that
does not disrupt melatonin levels and internal timekeeping.”