Table 2.
Association Between a Score for Night-Shift Work Based on a Job Exposure Matrixa and Breast Cancer Risk Among 73,049 Women Without Incident Cancer at Baseline, Shanghai Women's Health Studyb
| No. in Total Cohort | No. of Cases | Adjusted for Age |
Fully Adjustedc |
|||
| HR | 95% CI | HR | 95% CI | |||
| Ever working night shifts | ||||||
| Never | 41,164 | 423 | 1 | 1 | ||
| Ever | 31,885 | 294 | 0.9 | 0.8, 1.1 | 1.0 | 0.9, 1.2 |
| Average night-shift score | ||||||
| 0 | 41,164 | 423 | 1 | 1 | ||
| >0–≤1.29 | 10,507 | 102 | 1.0 | 0.8, 1.2 | 1.0 | 0.8, 1.2 |
| >1.29–≤2.38 | 10,882 | 109 | 1.0 | 0.8, 1.2 | 1.1 | 0.9, 1.3 |
| >2.38 | 10,496 | 83 | 0.8 | 0.6, 1.0 | 0.9 | 0.7, 1.2 |
| P-trend | 0.09 | 0.73 | ||||
| Duration of working in jobs with score >0, years | ||||||
| 0 | 41,164 | 423 | 1 | 1 | ||
| >0–≤14 | 10,569 | 108 | 1.0 | 0.8, 1.3 | 1.1 | 0.9, 1.3 |
| >14–≤25 | 11,056 | 89 | 0.8 | 0.6, 1.0 | 0.9 | 0.7, 1.1 |
| >25 | 10,260 | 97 | 0.9 | 0.7, 1.1 | 1.0 | 0.8, 1.3 |
| P-trend | 0.12 | 0.72 | ||||
| Lifetime cumulative night-shift score | ||||||
| 0 | 41,164 | 423 | 1 | 1 | ||
| >0–≤34 | 10,530 | 102 | 1.0 | 0.8, 1.2 | 1.0 | 0.8, 1.3 |
| >34–≤66 | 11,473 | 103 | 0.9 | 0.7, 1.1 | 1.0 | 0.8, 1.2 |
| >66 | 9,882 | 89 | 0.9 | 0.7, 1.1 | 1.0 | 0.8, 1.2 |
| P-trend | 0.15 | 0.84 | ||||
| Age at starting first job with score >0, years | ||||||
| Never did shift work | 41,164 | 423 | 1 | 1 | ||
| >26 | 10,055 | 87 | 0.8 | 0.7, 1.1 | 1.0 | 0.8, 1.2 |
| >20–≤26 | 9,721 | 98 | 1.0 | 0.8, 1.2 | 1.0 | 0.8, 1.3 |
| >0–≤20 | 12,109 | 109 | 0.9 | 0.7, 1.1 | 1.0 | 0.8, 1.2 |
Abbreviations: CI, confidence interval; HR, hazard ratio.
Each occupation in the occupational histories of the study participants was scored: 0 = no night-shift work; 1 = incidental night-shift work; 2 = jobs likely to involve the night shift, but entailing working only part of the night or being on call; and 3 = jobs probably involved all-night shifts. Lifetime cumulative scores were calculated by multiplying the years spent in each job by the score for that particular job, and then summing the product values over all jobs in a subject's work history. The average score was calculated by dividing this sum by the total years of employment.
Follow-up extended from baseline (1996–2000) to December 2007, death, or the first diagnosis of incident cancer.
Adjusted for age, education, family history of breast cancer, number of pregnancies, age at first birth, and occupational physical activity.