Table 1.
Age-Adjusted Baseline Characteristics by Categories of Rotating Night-Shift Work Duration in the Nurses' Health Study (n = 78,516), 1988, and Nurses' Health Study II (n = 114,559), 1989
| Characteristic | Years of Rotating Night-Shift Work by Study | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| NHS | NHS2 | |||||||||||||||
| None (n = 31,746) | 1–14 (n = 40,966) | 15–29 (n = 4,424) | ≥30 (n = 1,380) | None (n = 43,529) | 1–9 (n = 65,783) | 10–19 (n = 5,085) | ≥20 (n = 162) | |||||||||
| Mean (SD)a | %a | Mean (SD)a | %a | Mean (SD)a | %a | Mean (SD)a | %a | Mean (SD)a | %a | Mean (SD)a | %a | Mean (SD)a | %a | Mean (SD)a | %a | |
| Age, years | 54.3 (7.2) | 54.7 (7.1) | 56.1 (6.9) | 60.4 (4.6) | 34.8 (4.7) | 34.6 (4.7) | 37.2 (3.4) | 41.0 (2.4) | ||||||||
| Height, inches | 64.5 (2.4) | 64.5 (2.4) | 64.4 (2.5) | 64.5 (2.5) | 64.9 (2.6) | 64.9 (2.6) | 64.9 (2.7) | 63.9 (2.9) | ||||||||
| BMIb | 25.3 (4.8) | 25.6 (4.9) | 27.0 (5.5) | 26.6 (5.2) | 23.9 (4.9) | 24.1 (5.1) | 25.3 (5.9) | 24.8 (5.8) | ||||||||
| BMIb at age 18 years | 21.2 (2.9) | 21.3 (3.0) | 21.9 (3.4) | 21.9 (3.7) | 21.2 (3.2) | 21.3 (3.4) | 22.0 (4.1) | 21.3 (4.2) | ||||||||
| Childhood body sizec | 2.4 (1.3) | 2.4 (1.3) | 2.4 (1.4) | 2.3 (1.4) | 2.6 (1.2) | 2.6 (1.2) | 2.7 (1.3) | 2.6 (1.3) | ||||||||
| Adolescent body sizec | 2.7 (1.2) | 2.7 (1.2) | 2.7 (1.3) | 2.7 (1.3) | 2.9 (1.1) | 2.9 (1.1) | 3.0 (1.2) | 2.7 (1.2) | ||||||||
| Menarche before age 12 years | 22 | 23 | 24 | 30 | 24 | 25 | 29 | 35 | ||||||||
| Ever use of OCs | 49 | 49 | 46 | 44 | 83 | 83 | 83 | 57 | ||||||||
| Nulliparous | 5 | 6 | 6 | 6 | 28 | 32 | 36 | 42 | ||||||||
| No. of childrend | 3.2 (1.5) | 3.1 (1.5) | 3.2 (1.6) | 3.2 (1.6) | 2.1 (0.9) | 2.0 (0.9) | 2.1 (0.9) | 2.0 (0.7) | ||||||||
| Age at first birth, yearsd | 24.9 (3.2) | 25.3 (3.4) | 24.9 (3.5) | 25.3 (3.1) | 25.2 (4.0) | 25.7 (4.1) | 25.3 (4.1) | 23.0 (3.5) | ||||||||
| Ever breastfedd | 47 | 49 | 47 | 43 | 48 | 46 | 39 | 32 | ||||||||
| Postmenopausal | 67 | 68 | 70 | 86 | 2 | 2 | 3 | 4 | ||||||||
| Menopause because of surgerye | 41 | 42 | 44 | 40 | 93 | 92 | 96 | 88 | ||||||||
| Current MHT usee | 35 | 35 | 29 | 29 | 83 | 79 | 84 | 82 | ||||||||
| Age at menopause, yearse | 48.8 (4.8) | 48.7 (4.8) | 48.3 (4.7) | 48.4 (4.3) | 37.7 (4.3) | 37.5 (4.7) | 37.4 (3.6) | 40.4 (0.8) | ||||||||
| First-degree family history of breast cancer | 11 | 11 | 11 | 12 | 6 | 6 | 5 | 2 | ||||||||
| History of benign breast disease | 37 | 38 | 34 | 30 | 28 | 29 | 27 | 17 | ||||||||
| Current smoker | 17 | 19 | 25 | 25 | 12 | 13 | 19 | 23 | ||||||||
| Pack-years of smokingf | 23.1 (19.5) | 23.2 (19.4) | 26.1 (20.0) | 26.2 (20.0) | 11.4 (8.2) | 11.3 (8.2) | 11.8 (8.3) | 12.3 (7.6) | ||||||||
| Alcohol consumption, g/day | 6.1 (10.6) | 6.3 (10.7) | 5.3 (10.5) | 5.5 (9.7) | 3.0 (6.0) | 3.2 (6.1) | 2.9 (6.1) | 1.3 (4.4) | ||||||||
| Physical activity, MET-hours/week | 14.6 (20.8) | 16.0 (21.9) | 16.1 (21.7) | 19.3 (28.3) | 22.7 (34.2) | 26.0 (37.9) | 32.8 (48.4) | 25.7 (56.2) | ||||||||
| Nurse's educational level of bachelor's or higherg | 31 | 30 | 24 | 22 | N/A | N/A | N/A | N/A | ||||||||
| Husband's educational level of college or higherh | 55 | 56 | 42 | 49 | 80 | 83 | 80 | 90 | ||||||||
| Ever had a mammogram | 77 | 76 | 70 | 72 | 38 | 37 | 34 | 29 | ||||||||
Abbreviations: BMI, body mass index; MET, metabolic equivalent; MHT, menopausal hormone therapy; N/A, not applicable; NHS, Nurses' Health Study; NHS2, Nurses' Health Study II; OC, oral contraceptive.
a Values are standardized to the age distribution of the study population.
b Weight (kg)/height (m)2.
c Body size was recalled using pictures of body outlines that were numbered 1–9 from leanest to fattest.
d Among parous women only.
e Among postmenopausal women only.
f Among smokers only.
g Collected in 1992 for NHS only.
h Among married or widowed women only (collected in 1992 for NHS; collected in 1999 for NHS2).