Key Points

• 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.