Results from a new study in laboratory mice show that night time exposure to artificial light stimulated the growth of human breast tumours by suppressing the levels of a key hormone called melatonin. The study also showed that extended periods of night time darkness greatly slowed the growth of these tumours.

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 industrialised countries like the United States, states a NIH release.

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.

“This is the first experimental evidence that artificial light plays an integral role in the growth of human breast cancer. This finding will enable scientists to develop new strategies for evaluating the effects of light and other environmental factors on cancer growth,” said NIEHS director David A. Schwartz.

“The risk of developing breast cancer is about five times higher in industrialised nations than it is in underdeveloped countries. These results suggest that the increasing night time use of electric lighting, both at home and in the workplace, may be a significant factor,.” said Les Reinlib, a programme administrator with the NIEHS’ grants division.

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 tumours.

“We know that many tumours are largely dependent on a nutrient called linoleic acid, an essential fatty acid, in order to grow,” said David Blask, a neuroendocrinologist with the Bassett Research Institute and lead author on the study.

“Melatonin interferes with the tumour’s ability to use linoleic acid as a growth signal, which causes tumour metabolism and growth activity to shut down,” added Blask.

To test this hypothesis, the researchers injected human breast cancer cells into laboratory mice. Once these cells developed into cancerous tumours, the tumours were implanted into female rats where they could continue to grow and develop.

The researchers took blood samples from 12 healthy, premenopausal volunteers. The samples were collected under three different conditions, during daytime and night time following 2 hours of complete darkness, and during the night time following 90 minutes of exposure to bright fluorescent light. These blood samples were then pumped directly through the developing tumours.

“The melatonin-rich blood collected from subjects while in total darkness severely slowed the growth of the tumours. These results are due to a direct effect of the melatonin on the cancer cells. The melatonin is clearly suppressing tumour development and growth,” said Blask.

In contrast, tests with the melatonin-depleted blood from light-exposed subjects stimulated tumour 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 favour of tumour 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. This is because melatonin concentrations are not elevated during a person’s normal waking hours,” said Mark Rollag, a visiting research scientist at the University of Virginia and one of the study co-authors.

“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. This might include lighting with a wavelength and intensity that does not disrupt melatonin levels and internal timekeeping,” said Richard Stevens, an epidemiologist with the University of Connecticut Health Centre who has authored several papers on the subject.

“Day workers who spend their time indoors would benefit from lighting that better mimics sunlight. Companies that employ shift workers could introduce lighting that allows the workers to see without disrupting their circadian and melatonin rhythms,” added Stevens.

NIEHS, a component of the National Institutes of Health, supports research to understand the effects of the environment on human health.