Research table: Night shift work, light at night 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 for many years, such as nurses and flight attendants, may have a slightly increased risk of breast cancer.

One possible reason for the increased risk of breast cancer is the exposure to light at night 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)
Finnish Public Sector cohort [6]	33,359 pre- and postmenopausal women (1,129 cases)		10-13	Ever vs. never	1.22 (0.80-1.85)
Shanghai Women’s Health Study [7]	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. [8-9]	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)
Meta-analyses
Van NTH et al. [10]	15 cohort studies			Ever vs. never	0.98 (0.93-1.03)
	4 nested case-control studies			Ever vs. never	1.14 (0.89-1.46)
Manouchehri et al. [11]	7 cohort studies			Less than 10 years vs. never	1.02 (0.97-1.06)
				10 or more years vs. never	1.01 (0.95-1.07)
	6 nested case-control studies			Less than 10 years vs. never	1.40 (0.90-2.19)
				10 or more years vs. never	1.5 (0.86-2.66)

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

† Estimated from person-years

‡ Findings for 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 years or more 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 they 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. Härmä M, Ojajärvi A, Koskinen A, Lie JA, Hansen J. Shift work with and without night shifts and breast cancer risk in a cohort study from Finland. Occup Environ Med. 80(1):1-6, 2023.
7. 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.
8. 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.
9. Lie JA, Roessink J, Kjærheim K. Breast cancer and night work among Norwegian nurses. Cancer Causes Control. 17(1):39-44, 2006.
10. Van NTH, Hoang T, Myung SK. Night shift work and breast cancer risk: a meta-analysis of observational epidemiological studies. Carcinogenesis. 42(10):1260-1269, 2021.
11. Manouchehri E, Taghipour A, Ghavami V, Ebadi A, Homaei F, Latifnejad Roudsari R. Night-shift work duration and breast cancer risk: an updated systematic review and meta-analysis. BMC Womens Health. 21(1):89, 2021.