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SCIENCE NEWS: Study links breast cancer to light at night

Published 25 Aug 2017

A new US study has found that some women who live in areas with higher levels of outdoor light at night may be at increased risk of breast cancer, compared to those living in areas with lower levels.

Blog by Science Policy Officer, Dr Margaret Wexler


Researchers at the Harvard T.H. Chan School of Public Health found exposure to cumulative light at night (LAN) increased the risk of invasive breast cancer, but only in premenopausal women who are current or former smokers (1). The overall increase in breast cancer risk was estimated to be 14% for those exposed to the highest levels of outdoor LAN, compared to those exposed to the lowest levels.

Time-varying satellite data was used to estimate LAN exposure at residential addresses of each of the 110,000 study participants and these data were correlated with occurrence of breast cancer and other parameters linked to breast cancer risk.

Although previous studies have shown that women who live in areas with high LAN are at increased risk of breast cancer, this new, comprehensive study used data collected over 24 years and included women enrolled in the U.S. nationwide Nurses’ Health Study II (2).

The current study found the association between LAN and breast cancer was more pronounced among night shift workers. Numerous studies have shown that shift work is a risk factor for breast cancer in women (3).

Why might light at night affect breast cancer risk?

Light at night and night shift work reduce the level of melatonin in the bloodstream. This hormone plays a central role in regulating the body's sleep cycle, or circadian rhythm, and its production peaks at night. Light at night inhibits release of melatonin from the pineal gland, located in the brain. Melatonin is protective against several cancers, including breast cancer. It inhibits the development, promotion, and progression of breast cancer and is anti-oestrogenic (4). Women with breast cancer, especially oestrogen receptor positive breast cancer, have lower plasma levels of melatonin (5). Furthermore, low urinary melatonin levels may be associated with increased breast cancer risk (6).

Is smoking a risk factor for breast cancer?

Numerous studies have attempted to discover whether smoking increases breast cancer risk, with varying results.  Most of the more recent studies and meta-studies conclude that smoking does increase risk, but only modestly (e.g. 7). Interestingly though, this report showed an association between LAN and breast cancer risk was found only in current or past smokers who are premenopausal. Another study (which also included Nurses’ Health Study participants) found melatonin levels are lower in smokers compared to those who never smoked (8). Therefore, it seems plausible that reduced levels of melatonin in smokers in combination with lower levels resulting from LAN act in combination to increase breast cancer risk.

This current study is a good example of how environmental parameters - in this case outdoor light levels at night and smoking history - interact, resulting in an increase in breast cancer incidence in a sub-population – premenopausal women. 

What should I do to reduce my risk?

It is a good idea to try and sleep in a completely darkened room. Try to reserve the bedroom for sleeping, don’t sleep with the lights on and avoid tech or electrical equipment that gives out any light. If you live in an area where street lighting is used, or work shifts and need to sleep in the day, consider investing in some good blackout blinds for the bedroom. 

For more tips and advice on getting a good night's sleep, visit the NHS sleep pages.

For help and advice on smoking, we recommend speaking to your GP or try one of your local Stop Smoking services.



1. James, P. et al. (2017). 'Outdoor Light at Night and Breast Cancer Incidence in the Nurses’ Health Study II. Environmental Health Perspectives, August 17, 2017, doi: 10.1289/EHP935.

2. Nurses’ Health study II.

3. Hansen, J. (2017). Night Shift Work and Risk of Breast Cancer. Current Environmental Health Reports Aug 2. doi: 10.1007/s40572-017-0155-y. [Epub ahead of print].

4. Hill, S. M. et al (2015). Melatonin: an inhibitor of breast cancer. Endocrine related Cancer 22 R183-R204.

5. Basler, M. et al. (2015). Urinary excretion of melatonin and association with breast cancer: meta-analysis and review of the literature. Breast Care 9(3): 182-187.

6. Basler et al (2015). Ibid

7. Macacu, A. et al. (2015). Active and passive smoking and risk of breast cancer: a meta-analysis. Breast Cancer Research and Treatment. 154(2): 213–224.

8. Schernhammer, E. S. et al. (2006). Urinary 6-sulfatoxymelatonin levels and their correlations with lifestyle factors and steroid hormone levels. Journal of Pineal Research 40: 16–124.

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