According to American researchers women who are exposed to too much light at night could suffer a disruption of crucial hormones and raise their risk of developing breast tumours.
The researchers say that women who work night shifts such as nurses and air stewardesses put themselves at particular risk.
It has been suspected for some time that long exposure to artificial light could be cancerous and this new study suggests that this could to some extent explain the rising levels of breast cancers in rich countries, where the risk of developing cancer is five times higher than in underdeveloped countries.
Breast cancer is the most common cancer in women and affects one in nine women at some point in their lives.
Thousands of women are diagnosed each year with the disease and each month worldwide thousands die from the disease.
The study by American National Cancer Institute and National Institute of Environmental Health Sciences demonstrates that artificial light increased the risk of developing breast cancer by inhibiting the levels of the key hormone melatonin.
Melatonin is secreted by the pineal gland and this secretion takes place, in the main, at night.
The hormone regulates sleeping and waking cycles and it appears that the presence of light suppresses the production of the hormone.
In the study the researchers tested the theory by implanting human breast cancer cells in lab mice and blood samples taken from women were injected into the mice.
The samples were taken three times, during the day, in the early hours of the morning and at night after the women were exposed to artificial light.