Breast Cancer Risk Factors: Night Shift Work and Light at Night

Some studies show women who routinely work night shifts for many years have a small increased risk of breast cancer [279-282].

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

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

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

Learn about other factors linked to the risk of breast cancer.

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. Exposure to light at night disrupts this process.

Findings on melatonin levels in the body and breast cancer risk are mixed [283-284].

However, some findings suggest women with lower levels of melatonin may have a greater risk of breast cancer than women with higher levels [284]. Exposure to light at night may be linked to an increased risk of breast cancer because it limits the production of melatonin.

Lower melatonin levels may also be linked to an increased production of estrogen [281]. High levels of estrogen in the body are linked to an increased risk of breast cancer [22].

These processes in the body and how they may be related to breast cancer risk among night shift workers are under study.

Learn more about estrogen and breast cancer risk.

SUSAN G. KOMEN®‘S BREAST SELF-AWARENESS MESSAGES

1. Know your risk Talk with both sides of your family to learn about your family health history. Talk with a health care provider about your risk of breast cancer. 2. Get screened Talk with a health care provider about which screening tests are right for you if you’re at higher risk. Have a mammogram every year starting at age 40 if you’re at average risk.* Have a clinical breast exam at least every 3 years starting at age 25, and every year starting at age 40.* * Per National Comprehensive Cancer Network Guidelines 3. Know what is normal for you See a health care provider if you notice any of these breast changes: Lump, hard knot or thickening inside the breast or underarm area Swelling, warmth, redness or darkening of the breast Change in the size or shape of the breast Dimpling or puckering of the skin Itchy, scaly sore or rash on the nipple Pulling in of the nipple or other parts of the breast Nipple discharge that starts suddenly New pain in one spot that doesn’t go away To see illustrations of these warning signs, please visit the Warning Signs of Breast Cancer page. 4. Make healthy lifestyle choices Maintain a healthy weight. Make exercise part of your routine. Limit alcohol intake. Limit menopausal hormone therapy (postmenopausal hormone use). Breastfeed if you can.

Updated 12/17/24

This content is regularly reviewed by an expert panel including researchers, practicing clinicians and patient advocates.