The Who, What, Where, When and Sometimes, Why.

Light at Night and Shift Work

Some studies show women who routinely work night shifts for many years have a small increased risk of breast cancer [254-256].

More research is needed to understand which aspects of shift work may impact breast cancer risk.

One possible reason for the increased risk among these workers is their exposure to light at night.

Light at night, melatonin and breast cancer

Melatonin is a hormone in the body that helps control the sleep cycle. During the night, the body makes melatonin and exposure to light at night disrupts this process.

Findings on melatonin levels in the body and breast cancer risk are mixed [257-258].

However, some findings suggest women with lower levels of melatonin may have a higher risk of breast cancer than women with higher levels [258]. Exposure to light at night may be linked to an increased risk of breast cancer because it limits the production of melatonin.

Lowering melatonin levels also can increase the production of estrogen. High levels of estrogen in the body are linked to an increased risk of breast cancer [19].

These processes in the body and their effect on breast cancer risk among night shift workers are under study.

Learn more about estrogen and breast cancer risk.


For a summary of research studies on light at night and shift work and breast cancer, visit the Breast Cancer Research Studies section.

Updated 02/22/21


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