Breast cancerExposure to sunlight

There is evidence that sunlight affects risk of breast cancer by affecting melatonin levels.

More recent ecological studies provided more supporting evidence and included indices for other breast cancer risk-modifying factors in order to more accurately assess the role of UVB and vitamin D in the United States1 2 3 and Spain4 and France5 6.

A cross-sectional study using data from National Health and Nutrition Examination Survey Epidemiologic Follow-up Study [NHANES Follow-up Study] found the risk reductions [RRs] were highest for women who lived in United States regions of high solar radiation, with RRs ranging from 0.35-0.75 7.

A death certificate-based case-control study of mortality found significant negative association with jobs with the highest occupational exposure to sunlight (odds ratio (OR) 0.82 (95% confidence interval (CI) 0.70-0.97) for female breast cancer8

A case-control study from Ontario, Canada found: “Time spent outdoors was associated with reduced breast cancer risk during 4 periods of life (>21 vs. ≤6 hours/week age-adjusted odds ratio (OR) = 0.71, 95% confidence interval (CI): 0.60, 0.85 in the teenage years; OR = 0.64, 95% CI: 0.53, 0.76 in the 20s-30s; OR = 0.74, 95% CI: 0.61, 0.88 in the 40s-50s; and OR = 0.50, 95% CI: 0.37, 0.66 in the 60s-74 years).”9

Note that ecological studies integrate the effect of risk-modifying factors such as solar UVB irradiance and vitamin D throughout the entire lifetime while observational studies generally look at a period of time 3-15 after drawing blood for a serum 25(OH)D value.

There is also evidence that sunlight affects risk of breast cancer by affecting melatonin levels. During the day when there is sufficient blue sunlight, no melatonin is produced. At night, when there is no bright blue light, melatonin is produced and helps induce sleep.

Several studies had reported that melatonin reduces the risk of breast cancer10 11. A recent paper summarized the global data for breast cancer incidence seasonality, finding peaks in spring and fall at mid- and high latitudes12. It was suggested that vitamin D reduces growth of breast cancer in summer and melatonin does so in winter.

Page last edited: 22 August 2011


  1. Grant, W. B. An ecologic study of dietary and solar ultraviolet-B links to breast carcinoma mortality rates. Cancer. 2002 Jan 1; 94 (1): 272-81.
  2. Grant, W. B. Lower vitamin-D production from solar ultraviolet-B irradiance may explain some differences in cancer survival rates. J Natl Med Assoc. 2006 Mar; 98 (3): 357-64.
  3. Grant, W. B. Garland, C. F. The association of solar ultraviolet B (UVB) with reducing risk of cancer: multifactorial ecologic analysis of geographic variation in age-adjusted cancer mortality rates. Anticancer Res. 2006 Jul-Aug; 26 (4A): 2687-99.
  4. Grant, W. B. An ecologic study of cancer mortality rates in Spain with respect to indices of solar UVB irradiance and smoking. Int J Cancer. 2007 Mar 1; 120 (5): 1123-8.
  5. Engel, P. Fagherazzi, G. Mesrine, S. Boutron-Ruault, M. C. Clavel-Chapelon, F. Joint Effects of Dietary Vitamin D and Sun Exposure on Breast Cancer Risk: Results from the French E3N Cohort. Cancer Epidemiol Biomarkers Prev. 2011 Jan; 20 (1): 187-98.
  6. Grant, W. B. An ecological study of cancer incidence and mortality rates in France with respect to latitude, an index for vitamin D production. Dermatoendocrinol. 2010 Apr; 2 (2): 62-7.
  7. John, E. M. Schwartz, G. G. Dreon, D. M. Koo, J. Vitamin D and breast cancer risk: the NHANES I Epidemiologic follow-up study, 1971-1975 to 1992. National Health and Nutrition Examination Survey. Cancer Epidemiol Biomarkers Prev. 1999 May; 8 (5): 399-406.
  8. Freedman, D. M. Dosemeci, M. McGlynn, K. Sunlight and mortality from breast, ovarian, colon, prostate, and non-melanoma skin cancer: a composite death certificate based case-control study. Occup Environ Med. 2002 Apr; 59 (4): 257-62.
  9. Anderson, L. N. Cotterchio, M. Kirsh, V. A. Knight, J. A. Ultraviolet Sunlight Exposure During Adolescence and Adulthood and Breast Cancer Risk: A Population-based Case-Control Study Among Ontario Women. American journal of epidemiology. 2011 Jun 9;
  10. Schernhammer, E. S. Hankinson, S. E. Urinary melatonin levels and postmenopausal breast cancer risk in the Nurses' Health Study cohort. Cancer Epidemiol Biomarkers Prev. 2009 Jan; 18 (1): 74-9.
  11. Stevens, R. G. Light-at-night, circadian disruption and breast cancer: assessment of existing evidence. Int J Epidemiol. 2009 Aug; 38 (4): 963-70.
  12. Oh, E. Y. Ansell, C. Nawaz, H. Yang, C. H. Wood, P. A. Hrushesky, W. J. Global breast cancer seasonality. Breast Cancer Res Treat. 2010 Aug; 123 (1): 233-43.