Table 3.
Associations of Duration of Rotating Night-Shift Work and Invasive Breast Cancer, Stratified by Follow-up Period, in the Nurses' Health Study, 1988–2012, and the Nurses' Health Study II, 1989–2013
| Exposure Measure | Follow-up ≤10 Years | Follow-up >10 Years | Multivariable P for Interaction | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| No. of Cases | No. of Person-Years | Age Adjusted | Multivariable Adjusteda | No. of Cases | No. of Person-Years | Age Adjusted | Multivariable Adjusteda | ||||||||
| HR | 95% CI | HR | 95% CI | P for Trend | HR | 95% CI | HR | 95% CI | P for Trend | ||||||
| NHS rotating night-shift work history, years | 2,598 | 735,599 | 0.04 | 3,373 | 819,920 | 0.25 | 0.03 | ||||||||
| None | 977 | 298,701 | 1.00 | Referent | 1.00 | Referent | 1,405 | 336,729 | 1.00 | Referent | 1.00 | Referent | |||
| 1–14 | 1,415 | 383,622 | 1.11 | 1.02, 1.21 | 1.09 | 1.00, 1.18 | 1,747 | 427,392 | 0.97 | 0.91, 1.05 | 0.96 | 0.89, 1.03 | |||
| 15–29 | 146 | 40,739 | 1.03 | 0.86, 1.23 | 1.07 | 0.90, 1.28 | 185 | 43,381 | 1.01 | 0.87, 1.18 | 1.05 | 0.90, 1.23 | |||
| ≥30 | 60 | 12,537 | 1.23 | 0.95, 1.60 | 1.26 | 0.97, 1.64 | 36 | 12,418 | 0.65 | 0.47, 0.91 | 0.68 | 0.49, 0.95 | |||
| NHS2 1989 baseline rotating night-shift work history, years | 1,116 | 1,084,864 | 0.71 | 2,454 | 1,452,478 | 0.24 | 0.85 | ||||||||
| None | 416 | 412,724 | 1.00 | Referent | 1.00 | Referent | 902 | 553,730 | 1.00 | Referent | 1.00 | Referent | |||
| 1–9 | 637 | 622,782 | 1.03 | 0.91, 1.17 | 1.02 | 0.90, 1.15 | 1,434 | 833,620 | 1.07 | 0.98, 1.16 | 1.07 | 0.98, 1.16 | |||
| 10–19 | 57 | 47,867 | 0.94 | 0.71, 1.24 | 0.96 | 0.73, 1.27 | 111 | 63,327 | 0.94 | 0.77, 1.14 | 1.01 | 0.83, 1.24 | |||
| ≥20 | 6 | 1,491 | 2.13 | 0.95, 4.80 | 2.35 | 1.04, 5.31 | 7 | 1,801 | 1.63 | 0.77, 3.45 | 1.95 | 0.92, 4.15 | |||
| NHS2 cumulative rotating night-shift work (updated), yearsb | 1,034 | 977,132 | 0.75 | 2,154 | 1,213,546c | 0.89 | 0.73 | ||||||||
| None | 341 | 321,600 | 1.00 | Referent | 1.00 | Referent | 609 | 346,804 | 1.00 | Referent | 1.00 | Referent | |||
| 1–9 | 621 | 602,095 | 0.98 | 0.86, 1.12 | 0.97 | 0.85, 1.11 | 1,381 | 767,303 | 1.06 | 0.96, 1.16 | 1.07 | 0.97, 1.18 | |||
| 10–19 | 60 | 50,481 | 0.92 | 0.70, 1.21 | 0.94 | 0.71, 1.23 | 141 | 88,801 | 0.90 | 0.74, 1.07 | 0.95 | 0.79, 1.14 | |||
| ≥20 | 12 | 2,956 | 1.99 | 1.11, 3.56 | 2.13 | 1.19, 3.81 | 23 | 10,637 | 1.10 | 0.72, 1.66 | 1.19 | 0.78, 1.81 | |||
Abbreviations: CI, confidence interval; HR, hazard ratio; NHS, Nurses' Health Study; NHS2, Nurses' Health Study II.
a Multivariable-adjusted models were adjusted for the following covariates: age (months), height (inches; continuous), body mass index (weight (kg)/height (m)2; <18.5, 18.5–24.9, 25.0–29.9, or ≥30), body mass index at age 18 years (<18.5, 18.5–24.9, 25.0–29.9, or ≥30), adolescent body size (average of diagram scores at ages 10 and 20 years; 1.0, 1.5–2.0, 2.5–3.0, 3.5–4.0, or ≥4.5), age at menarche (<12, 12–13, or ≥14 years), age at first birth and parity combined (for NHS: nulliparous, age <25 years and 1–2 children, age <25 years and ≥3 children, age 25–29 years and 1–2 children, age 25–29 years and ≥3 children, age ≥30 years and 1–2 children, or age ≥30 years and ≥3 children; for NHS2: nulliparous, parous age <25 years, parous age 25–29 years, or parous age ≥30 years), breastfeeding (for NHS: none, 1–11 months, or ≥12 months; for NHS2: none, 1–12 months, or >12 months), type of menopause and age at menopause combined (premenopausal, naturally postmenopausal at age <45 years, naturally postmenopausal at age ≥45 years, surgically postmenopausal at age <45 years, or , surgically postmenopausal at age ≥45 years), menopausal hormone therapy use (never, past, or current), duration of use of menopausal hormone therapy with estrogen alone (months; continuous), duration of use of estrogen and progesterone menopausal hormone therapy (months; continuous), first-degree family history of breast cancer (yes or no), history of benign breast disease (yes or no), alcohol consumption (0.0, 0.1–14.0, 14.1–28.0, or >28 g/day), physical activity level (≤8.0, 8.1–16.0, 16.1–24.0, or >24 metabolic equivalent-hours/week), and current mammography use (yes or no). All categorical covariates were included in models with missing indicators.
b In the NHS2, analyses using updated data on duration of shift work excluded participants during the cycles in which they were missing information on shift work exposure, resulting in fewer cases and person-years than in analyses using history of shift work reported at baseline in 1989.
c Values do not sum to the total because of rounding.