Table 2.

Percentage and absolute mammographic density by selected characteristics of the rotating night shift work

Night shift work characteristicsnPercentage of mammographic density Adjusteda mean (95%CI)Absolute density, cm2 Adjusteda mean (95%CI)
Current rotating night shift
 No31922.5 (20.8–24.3)21.8 (19.4–24.3)
 Yes32123.6 (21.9–25.4)23.9 (21.4–26.4)
P0.620.25
Total duration of night shift work, y
 ≤56422.5 (18.8–26.1)23.3 (17.8–28.8)
 >5–≤1516524.5 (22.1–27.0)23.8 (20.3–27.2)
 >15–≤2520621.5 (19.4–23.7)21.9 (18.8–24.9)
 >2520523.7 (21.4–26.0)23.1 (20.0–26.2)
Ptrend0.930.78
Total number of hours of night shift work by tertilesb
 I21424.0 (21.8–26.1)23.8 (20.7–26.9)
 II21422.3 (20.2–24.5)22.5 (19.5–25.5)
 III21223.0 (20.7–25.2)22.3 (19.3–25.4)
Ptrend0.490.49
Cumulative number of night shifts
 ≤99918523.8 (21.5–26.1)23.6 (20.3–26.9)
 1,000–1,99922522.5 (20.5–24.6)22.9 (19.9–25.8)
 2,000+23023.1 (20.9–25.2)22.3 (19.4–25.2)
Ptrend0.660.56
Average frequency (weighted by duration); no. of night shifts/mo
 ≤221424.2 (22.0–26.3)24.4 (21.3–27.5)
 >2–≤3.921322.3 (20.3–24.4)22.8 (19.8–25.8)
 >3.921322.7 (20.6–24.8)21.5 (18.6–24.4)
Ptrend0.330.18
Total duration of reported exposure to surgical lamp, y
 Day shifts (reference)55023.3 (21.9–24.6)23.0 (21.1–24.9)
 >0–≤103921.3 (16.8–25.9)21.1 (14.3–27.8)
 >105122.7 (18.4–26.9)23.2 (17.1–29.4)
Ptrend0.610.92
  • aAdjustment for age, BMI, menopausal status, age at menopause, age at menarche, smoking, and season of the year.

  • bI tertile, ≤13,056 hours; II tertile, >13,056 to ≤24,304 hours; and III tertile, >24,304 hours.