An additional like between Light At Night (LAN) and cancer has now been established by a new study.
Previous studies have shown that living in areas that have more night-time illumination make men more susceptible to prostate cancer and women to breast cancer.
University of Haifa researchers' hypothesis was that LAN harms production of melatonin, a hormone that is released from the pineal gland during the dark part of the 24h cycle and which is linked to the body's cyclical night-day activity and seasonality.
In the current study, four groups of lab mice injected with cancerous cells were examined.
One group was exposed to "long days" of 16 hours of light and 8 hours of darkness, simulating exposure to artificial light beyond the natural number of light hours in a day.
A second group was exposed to the same "long days" but were treated with melatonin.
A third group was exposed to "short days" of 8 light hours and 16 dark hours and a fourth group was exposed to the same "short days" but during the dark hours was exposed to a half-hour interval of light.
The results showed that cancerous growths in mice exposed to "short days" were smallest (0.85 cubic cm. average), while those mice exposed to the interval of LAN during dark hours had larger growths (1.84 cubic cm. average) and those exposed to "long days" even larger growths (5.92 cubic cm. average).
The study also discovered that suppression of melatonin definitely influences development of the tumour.
The size of tumour in mice exposed to "long days" but treated with melatonin was only 0.62 cubic cm. on average, which is not much different from the size of the growth in mice exposed to "short days".
The researchers said that their study results show that suppression of melatonin due to exposure to LAN is linked to the worrying rise in the number of cancer patients over the past few years.
"Exposure to LAN- disrupts our biological clock and affects the cyclical rhythm that has developed over hundreds of millions of evolutionary years that were devoid of LAN.
Light pollution as an environmental problem is gaining awareness around the world, and the World Health Organization's International Agency for Research on Cancer (IARC) has already classified working the night shift as a higher grade of cancer risk," the researchers noted.