This summary table contains detailed information about research studies. While viewing summary tables offers an informative glimpse at the science behind many breast cancer guidelines and recommendations, they should be viewed with some caution. There are a number of concepts you must understand to be able to successfully read and interpret research tables. To get some background information about understanding research tables, please see How to Read a Research Table.
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Introduction: There is a growing body of evidence that women who routinely work overnight shifts (such as nurses) over many years are at a slightly increased risk of breast cancer. This may be due to the exposure to light related to these types of job. Being exposed to light throughout the night affects some hormone functions in the body that may be related to breast cancer.
Find more information on 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, nested case-control studies and case-control studies with at least 200 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
RR (95% CI)
|
Prospective cohort studies
|
Nurses' Health Study [1]
|
78,562 postmenopausal women
(2,441 cases)
|
10
|
1-14 years*
vs.
never
|
1.08
(0.99-1.18)
|
| |
|
|
15-29 years*
vs.
never
|
1.08
(0.90-1.30)
|
| |
|
|
30 or more years* vs.
never
|
1.36
(1.04-1.78)
|
Nurses' Health Study II [2]
|
115,022
pre- and postmenopausal women
(1,352 cases)
|
12
|
1-9 years*
vs.
never
|
0.98
(0.87-1.10)
|
| |
|
|
10-19 years*
vs.
never
|
0.91
(0.72-1.16)
|
| |
|
|
20 or more years* vs.
never
|
1.79
(1.06-3.01)
|
Nested case-control studies
|
| |
Cases
|
Controls
|
|
|
|
Lie et al. [3]
|
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)
|
Case-control studies
|
| |
Cases
|
Controls
|
|
|
|
Hansen [4]
|
7,035
|
7,035
|
-
|
At least one job for 6 months or more
vs.
other†
|
1.5
(1.3-1.7)
|
Davis et al. [5]
|
813
|
793
|
-
|
At least one job‡
vs.
never
|
1.6
(1.0-2.5)
|
O'Leary et al. [6]
|
576
|
585
|
-
|
At least one job‡
vs.
never
|
0.55
(0.32-0.94)
|
Meta analyses
|
Megdal et al. [7]
|
13 studies
|
-
|
Any night shift work
vs.
never
|
1.48
(1.36-1.61)
|
* Rotating night shifts at least three nights a week per month.
† Compared women who worked in an occupation with at least 60 percent night work for at least half a year to other employed women.
‡ At least one job in the past 10 to 15 years that required an overnight shift.
References
1. Schernhammer ES, Laden F, Speizer FE, et al. Rotating night shifts and risk of breast cancer in women participating in the Nurses' Health Study. J Natl Cancer Inst. 93(20):1563-8, 2001.
2. Schernhammer ES, Kroenke CH, Laden F, Hankinson SE. Night work and risk of breast cancer. Epidemiology. 17(1):108-11, 2006.
3. Lie JA, Roessink J, Kjærheim K. Breast cancer and night work among Norwegian nurses. Cancer Causes Control. 17(1):39-44, 2006.
4. Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiology. 2001 Jan;12(1):74-7, 2001.
5. Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. J Natl Cancer Inst. 93(20):1557-62, 2001.
6. O'Leary ES, Schoenfeld ER, Stevens RG, et al. for the Electromagnetic Fields and Breast Cancer on Long Island Study Group. Shift work, light at night, and breast cancer on Long Island, New York. Am J Epidemiol. 164(4):358-66, 2006.
7. 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.
Updated 09/12/09
