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

Light at night or shift work and breast cancer risk

This summary table contains detailed information about research studies. Summary tables are a useful way to look at the science behind many breast cancer guidelines and recommendations. However, to get the most out of the tables, it’s important to understand some key concepts. Learn how to read a research table.

Introduction: Women who routinely work overnight shifts (such as nurses and flight attendants) for many years may have a slightly increased risk of breast cancer.

One possible reason is the exposure to light related to these types of jobs. Being exposed to light throughout the night affects some hormone functions in the body that may be related to breast cancer.

Learn more about light at night, shift work and breast cancer risk.

Learn about the strengths and weaknesses of different types of studies.  

See how this risk factor compares with other risk factors for breast cancer.

Study selection criteria: Prospective cohort studies and nested case-control studies with at least 500 breast cancer cases, and meta-analyses.

Table note: Relative risk above 1 indicates increased risk. Relative risk below 1 indicates decreased risk.

Study

Study Population
(number of participants)

Follow-up
(years)

Duration of Overnight Shift Work

Relative Risk of Breast Cancer in Women who Worked Overnight Shifts versus Women who Did Not,
RR (95% CI)

Prospective cohort studies

Nurses’ Health Study [1] 

78,516
postmenopausal women
(5,971 cases)

24

15-29 years*
vs.
never

1.06
(0.94-1.19)

 

 

 

30 or more years*
vs.
never

0.95
(0.77-1.17)

Million Women Study [2]

522,246 
postmenopausal women
(4,809 cases)

3†

Ever
vs.
never

1.00
(0.92-1.08)

Nurses’ Health Study II [1]

114,559
pre- and postmenopausal women
(3,570 cases)

24

15-29 years*
vs.
never

0.94
(0.81-1.10)‡

 

 

 

30 or more years*
vs.
never

1.40
(1.00-1.97)‡

Sister Study [3]

48,451
pre- and postmenopausal women
(3,191 cases)

9

More than 5 years
vs.
never

0.92
(0.82-1.03)§

Dutch Labor Force Surveys [4]

285,723
pre- and postmenopausal women
(2,531 cases)

7

20 or more years||
vs.
never

0.95
(0.62-1.45)

Generations Study cohort [5]

102,869
pre- and postmenopausal women
(2,059 cases)

10

30 or more years
vs.
never

1.27
(0.86-1.87)

Shanghai Women’s Health Study [6]

73,049
pre- and postmenopausal women
(717 cases)

9

14-25 years
vs.
never

0.9
(0.7-1.1)

   

More than 25 years
vs.
never

1.0
(0.8-1.3)

Nested case-control studies

 

Cases

Controls

   

Lie et al. [7-8]

699

895

 

1-11 years
vs.
never

1.2
(0.9-1.5)

   

 

12 or more years
vs.
never

1.3
(0.9-1.8)

 

537

2,143

 

1-14 years
vs.
never

0.95
(0.67-1.33)

   

 

15-29 years
vs.
never

1.29
(0.82-2.02)

   

 

30 or more years
vs.
never

2.21
(1.10-4.45)

Danish Nurses Association [9]

267

1,035

 

1-5 years
vs.
never 

1.5
(0.99-2.5)

    

5-10 years
vs.
never

2.3
(1.4-3.5)

    

10-20 years
vs.
never

1.9
(1.1-2.8)

 

   

20 or more years
vs.
never

2.1
(1.3-3.2)

Meta-analyses

Yuan et al. [10]

49 studies

 

Ever
vs.
never

1.32
(1.20-1.45)

Kamdar et al. [11]

15 studies

 

Ever
vs.
never

1.21
(1.00-1.47)

   

8 or more years
vs.
never

1.04
(0.92-1.18)

He et al. [12]

15 studies

 

Ever
vs.
never

1.19
(1.08-1.32)

Jia et al. [13]

13 studies

 

Ever
vs.
never

1.20
(1.08-1.33)

   

15 or more years
vs.
never

1.15
(1.03-1.29)

Megdal et al. [14]

13 studies

 

Ever
vs.
never

1.48
(1.36-1.61)

Travis et al. [2]

10 studies

 

Ever
vs.
never

0.99
(0.95-1.03)

 

 

 

20 or more years
vs.
never

1.01
(0.93-1.10)

Wang et al. [15]

10 studies

 

Ever
vs.
never

1.19
(1.05-1.35)

* Rotating night shifts at least 3 nights a week each month

† Estimated from person-years

‡ Findings for all women who reported rotating night shift work (at least 3 nights a week each month). Among women who reported rotating night shift work for 20 or more years when they entered the study (those who began working night shifts at a young age), relative risk was 2.15 (1.23-3.73).

§ All women in the Sister Study cohort had a sister diagnosed with breast cancer, but had not been diagnosed with breast cancer themselves at the start of the study.

|| Regular night shifts

References

  1. Wegrzyn LR, Tamimi RM, Rosner BA, et al. Rotating night shift work and risk of breast cancer in the Nurses’ Health Studies. Am J Epidemiol. 186(5):532-540, 2017.
  2. Travis RC, Balkwill A, Fensom GK, et al. Night shift work and breast cancer incidence: three prospective studies and meta-analysis of published studies. J Natl Cancer Inst. 108(12), 2016. 
  3. Sweeney MR, Sandler DP, Niehoff NM, White AJ. Shift work and working at night in relation to breast cancer incidence. Cancer Epidemiol Biomarkers Prev. 29(3):687-689, 2020.
  4. Koppes LL, Geuskens GA, Pronk A, Vermeulen RC, de Vroome EM. Night work and breast cancer risk in a general population prospective cohort study in The Netherlands. Eur J Epidemiol. 29(8):577-84, 2014. 
  5. Jones ME, Schoemaker MJ, McFadden EC, Wright LB, Johns LE, Swerdlow AJ. Night shift work and risk of breast cancer in women: the Generations Study cohort. Br J Cancer. 121(2):172-179, 2019.
  6. Pronk A, Ji BT, Shu XO, et al. Night-shift work and breast cancer risk in a cohort of Chinese women. Am J Epidemiol. 171(9):953-9, 2010.
  7. Lie JA, Kjuus H, Zienolddiny S, Haugen A, Stevens RG, Kjærheim K. Night work and breast cancer risk among Norwegian nurses: assessment by different exposure metrics. Am J Epidemiol. 173(11):1272-9, 2011.
  8. Lie JA, Roessink J, Kjærheim K. Breast cancer and night work among Norwegian nurses. Cancer Causes Control. 17(1):39-44, 2006.
  9. Hansen J, Stevens RG. Case-control study of shift-work and breast cancer risk in Danish nurses: Impact of shift systems. Eur J Cancer. 48(11):1722-9, 2012.
  10. Yuan X, Zhu C, Wang M, Mo F, Du W, Ma X. Night shift work increases the risks of multiple primary cancers in women: a systematic review and meta-analysis of 61 articles. Cancer Epidemiol Biomarkers Prev. 27(1):25-40, 2018.
  11. Kamdar BB, Tergas AI, Mateen FJ, Bhayani NH, Oh J. Night-shift work and risk of breast cancer: a systematic review and meta-analysis. Breast Cancer Res Treat. 138(1):291-301, 2013.
  12. He C, Anand ST, Ebell MH, Vena JE, Robb SW. Circadian disrupting exposures and breast cancer risk: a meta-analysis. Int Arch Occup Environ Health. 88:533-547, 2015.
  13. Jia Y, Lu Y, Wu K, et al. Does night work increase the risk of breast cancer? A systematic review and meta-analysis of epidemiological studies. Cancer Epidemiol. 37(3):197-206, 2013.
  14. Megdal SP, Kroenke CH, Laden F, Pukkala E, Schernhammer ES. Night work and breast cancer risk: a systematic review and meta-analysis. Eur J Cancer. 41(13):2023-32, 2005.
  15. Wang F, Yeung KL, Chan WC, et al. A meta-analysis on dose-response relationship between night shift work and the risk of breast cancer. Ann Oncol. 24(11):2724-32, 2013.  

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