|New Research Shows Artificial Light at Night Stimulates Breast
Cancer Growth in Laboratory Mice
Results from a new study in laboratory mice show that 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 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.
The National Cancer Institute and the National Institute of Environmental Health
Sciences, agencies of the federal National Institutes of Health, provided funding
to researchers at the Bassett Research Institute of the Mary Imogene Bassett
Hospital in Cooperstown, New York and The Thomas Jefferson University in Philadelphia,
Pa. The results are published in the December 1, 2005 issue of the scientific
journal Cancer Research.
“This is the first experimental evidence that artificial light plays an integral
role in the growth of human breast cancer,” said NIEHS Director David A. Schwartz,
M.D. “This finding will enable scientists to develop new strategies for evaluating
the effects of light and other environmental factors on cancer growth.”
“The risk of developing breast cancer is about five times higher in industrialized
nations than it is in underdeveloped countries,” said Les Reinlib, Ph.D., a program
administrator with the NIEHS’ grants division. “These results suggest that the
increasing nighttime use of electric lighting, both at home and in the workplace,
may be a significant factor.”
Previous research showed 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.
“We know that many tumors are largely dependent on a nutrient called linoleic
acid, an essential fatty acid, in order to grow,” said David Blask, M.D., Ph.D.,
a neuroendocrinologist with the Bassett Research Institute and lead author on
the study. “Melatonin interferes with the tumor’s ability to use linoleic acid
as a growth signal, which causes tumor metabolism and growth activity to shut
To test this hypothesis, the researchers injected human breast cancer cells
into laboratory mice. Once these cells developed into cancerous tumors, the tumors
were implanted into female rats where they could continue to grow and develop.
The researchers then took blood samples from 12 healthy, premenopausal volunteers.
The samples were collected under three different conditions — during the
daytime, during the nighttime following 2 hours of complete darkness, and during
the nighttime following 90 minutes of exposure to bright fluorescent light. These
blood samples were then pumped directly through the developing tumors.
“The melatonin-rich blood collected from subjects while in total darkness severely
slowed the growth of the tumors. “These results are due to a direct effect of
the melatonin on the cancer cells,” said Blask. “The melatonin is clearly suppressing
tumor development and growth.”
In contrast, tests with the melatonin-depleted blood from light-exposed subjects
stimulated tumor growth. “We observed rapid growth comparable to that seen with
administration of daytime blood samples, when tumor activity is particularly
high,” Blask said.
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
“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.”
“Day workers who spend their time indoors would benefit from lighting that better
mimics sunlight,” added Stevens. “Companies that employ shift workers could introduce
lighting that allows the workers to see without disrupting their circadian and
NIEHS, a component of the National Institutes of Health, supports research
to understand the effects of the environment on human health. For more information
on breast cancer and other environmental health topics, visit our website at http://www.niehs.nih.gov/.
The National Institutes of Health (NIH) — The Nation's Medical Research
Agency — includes 27 Institutes and Centers and is a component of
the U. S. Department of Health and Human Services. It is the primary Federal
agency for conducting and supporting basic, clinical, and translational medical
research, and it investigates the causes, treatments, and cures for both common
and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov.