Disruptions to a person's natural body clock, such as being exposed to artificial light or staying up for long periods at night, increase the risk of breast cancer, according to a study by the American National Cancer Institute and the National Institute of Environmental Health Sciences.
The researchers used mice that had been implanted with human breast cancer cells, and injected blood samples from women into the mice. The blood samples were taken at three different times: during the day, early morning, and at night after exposure to artificial light.
While the blood taken at night slowed tumor growth by 80 percent, blood taken after exposure to artificial light increased cancer growth.
The findings represent the first proof that light is a risk factor for cancer. It is thought to act by inhibiting levels of the hormone melatonin, which is secreted by the pineal gland mostly at night and is an important variable in regulating a person's sleeping and waking cycles.