• Throughout the world, artificial light from many sources, including street lamps and brightly lit buildings, disrupts natural darkness at night.
• The light-at-night (LAN) theory proposes that exposure to artificial light slows the production of melatonin, a hormone normally produced at night, and that this reduction in melatonin disrupts our circadian rhythms—the pattern of changes in wakefulness, body temperature, heart rate, insulin production, and other biological processes.
• Research on shift workers such as nurses and flight attendants has found an increased risk of breast cancer in women who work at night for many years.
• The World Health Organization’s International Agency for Research on Cancer listed “shift work that involves circadian disruption” as “probably carcinogenic” in 2007, a decision that has led to much debate in the scientific community over concerns about the validity and quality of available evidence.
• Further research is needed to determine whether there is a link between exposure to light at night and breast cancer.