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Regular exposure to higher levels of outdoor light at night may increase the risk of developing breast cancer. The link was stronger among women who worked at night, exposed to light at night and night shift work contribute to breast cancer risk through circadian disruption.
Lighting during the night can disrupt circadian rhythms, reducing circulating melatonin levels and resetting of the circadian. Reduced melatonin may increase breast cancer risk, increased estrogen production and altered estrogen receptor function.
Melatonin is a hormone that plays a role in regulating the body’s sleep cycle. Melatonin production is high at night and is lower during the day. When women work at night or if they’re exposed to external light at night, their melatonin levels tend to stay low.
As levels of outdoor light at night increased, so did breast cancer rates.
The association between outdoor light at night and breast cancer was found among women who were premenopausal and those who were current or past smokers.
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