Table 1

Characteristics of study population

 Cases (n=1134)Controls (n=1179)Never night shift workers (n=1524)Ever night shift workers (n=789)
Mean (SD)/n (%)Mean (SD)/n (%)Mean (SD)/n (%)Mean (SD)/n (%)
Age57.30 (10.3)56.72 (10.0)58.0 (10.3)55.1 (9.6)
Body mass index25.68 (5.5)25.28 (5.8)25.3 (5.4)25.8 (6.2)
Ethnicity
 European707 (62.4)921 (78.3)1043 (68.5)585 (74.2)
 Chinese239 (21.1)114 (9.7)262 (17.2)91 (11.6)
 South Asian34 (3.0)35 (3.0)55 (3.6)14 (1.8)
 Filipino60 (5.3)38 (3.2)57 (3.7)41 (5.2)
 Japanese24 (2.1)14 (1.2)23 (1.5)15 (1.9)
 Mixed19 (1.7)12 (1.0)17 (1.1)14 (1.8)
 Other51 (4.5)43 (3.7)66 (4.3)28 (3.5)
Household income
 <$30 000203 (17.9)121 (10.3)217 (14.2)107 (13.6)
 $30 000–$59 999279 (24.6)269 (22.8)355 (23.3)193 (24.5)
 $60 000–$99 999245 (21.6)288 (24.4)334 (21.9)199 (25.2)
 >$100 000238 (21.0)339 (28.8)372 (24.4)205 (26.0)
 Not stated169 (14.9)162 (13.8))246 (16.1)85 (10.9)
Education
 High school or less392 (34.8)300 (25.5)499 (33.0)193 (24.5)
 College/trade certificate344 (30.6)349 (29.7)434 (28.7)259 (32.9)
 Undergraduate degree264 (23.5)302 (25.7)356 (23.5)210 (26.7)
 Graduate/professional degree126 (11.2)225 (19.1)225 (14.9)126 (16.0)
Menopausal status
 Premenopausal391 (34.6)445 (37.8)492 (32.4)344 (43.7)
 Postmenopausal739 (65.4)732 (62.2)1028 (67.6)443 (56.3)
Medication use
 Fertility drugs (ever/never)63 (5.6)64 (5.4)86 (5.7)41 (5.2)
Number of years NSAID use
 None994 (87.7)992 (84.1)1328 (87.1)658 (83.4)
 <2.3455 (4.9)62 (5.3)69 (4.5)48 (6.1)
 2.34–8.5048 (4.2)61 (5.2)72 (4.7)37 (5.0)
 ≥8.5137 (3.3)64 (5.4)55 (3.6)46 (5.8)
 Number of years oral contraceptive use
 None558 (49.2)429 (36.4)695 (45.6)292 (37.0)
 <4.50192 (16.9)248 (21.0)277 (18.2)163 (20.7)
 4.50–10.00215 (19.0)276 (23.4)317 (20.8)174 (22.1)
 ≥10.01169 (14.9)226 (19.2)235 (15.4)160 (20.3)
Number of years antidepressant use
 None911 (80.3)916 (77.8)1229 (80.7)598 (75.8)
 <1.5074 (6.5)86 (7.3)102 (6.7)58 (7.4)
 1.50–7.5087 (7.7)87 (7.4)95 (6.2)79 (10.0)
 ≥7.5162 (5.5)89 (7.6)97 (6.4)54 (6.8)
Number of years HRT use
 None833 (73.5)854 (72.4)1112 (73.0)575 (72.9)
 <5.0091 (8.0)99 (8.4)120 (7.9)70 (8.9)
 5.00–12.00114 (10.1)120 (10.2)156 (10.2)78 (9.9)
 ≥12.0196 (8.5)106 (9.0)136 (8.9)66 (8.4)
Reproductive history
 Age at menarche12.89 (1.6)12.84 (1.5)12.9 (1.5)12.7 (1.6)
 Ever been pregnant937 (82.9)937 (79.6)1234 (81.2)640 (81.3)
 Age at first pregnancy*27.69 (5.4)27.54 (5.3)27.6 (5.4)27.6 (5.4)
 Breastfeeding (number of months)6.9 (9.8)7.9 (10.4)7.1 (10.0)7.9 (10.5)
 Number of pregnancies*2.28 (1.7)2.25 (1.7)2.3 (1.7)2.3 (1.7)
 Age at first mammogram44.72 (8.8)42.78 (7.5)44.4 (8.5)42 (7.6)
 Family history of breast cancer223 (19.7)168 (14.3)267 (17.5)124 (15.7)
Lifestyle characteristics
 Current smoker72 (6.4)71 (6.0)89 (5.9)54 (6.9)
 Pack-years smoking5.64 (12.1)5.29 (11.2)5.3 (11.8)5.7 (11.5)
Lifetime alcohol consumption (number of drinks/week)
 Teen1.05 (3.5)1.63 (4.0)1.1 (3.1)1.9 (4.7)
 20s2.24 (3.9)3.26 (6.1)2.5 (5.2)3.3 (5.0)
 30s2.91 (6.0)3.61 (5.8)3.0 (5.6)3.7 (6.4)
 40s3.10 (6.0)3.75 (6.1)3.3 (6.2)3.6 (5.7)
 50s2.76 (4.9)3.71 (6.1)3.2 (5.6)3.3 (5.3)
  • *Among parous women only.