Passage one
Women who live in areas with higher leve ls of outdoor light at night may beat higher risk for breast cancer，according to a study from Harvard T.H.Chan School Of Public Health.The link was stronger among women who worked night shifts.
Previous studies have suggested that exposure to light at night may lead to decreased levels of the homone melatonin，which can disrupt circadian rhythms—our internal ’clocks’ that govern sleepiness and alertness—and，in turn，lead to increased breast cancer risk.
The new study，the most comprehensive to date to examine possible links between outdoor light at night and breast cancer，looked at data from nearly 110，000 women enrolled in the Nurses ’Health Study II from 1989-2013.The researchers linked data from satellite images of Earth taken at night time to residential addresses for each study participant，and also considered the influence of night shift work.The study also factored indetailed in formation on a variety of health and socioeconomic factors among participants.
Women exposed to the highest levels of outdoor light at night—those in the top fift—had an estimated 14%increased risk of breast cancer during the study period，as compared with women in the bottom fifth of exposure，the researchers found.As levels of outdoor light at night increased，so did breast cancer rates.
However，the association between outdoor light at night and breast cancer was found only among women who were premenopausal and those who were current or past smokers.In addition the link was stronger among women who worked night shifts suggesting that exposure to light at night and night shift work contribute jointly to breast cancer risk.