Table 2.

Sleep duration and breast cancer risk in a cohort of Finnish women, 1976 to 1996

No. of women (n = 12,222)No. of breast cancersAge-adjusted HR (95% CI)Multivariable HR (95% CI) *
Sleep duration in 1975, follow-up 1976 to 1996, seven-category classification
    ≤4 h4410.73 (0.10-5.32)0.88 (0.11-6.91)
    5 h18350.81 (0.32-2.03)0.91 (0.36-2.32)
    6 h954170.66 (0.40-1.11)0.74 (0.44-1.26)
    7 h3,473680.84 (0.63-1.13)0.81 (0.60-1.10)
    8 h (reference)5,5801201.001.00
    9 h1,650250.73 (0.48-1.13)0.64 (0.40-1.02)
    ≥10 h33860.82 (0.36-1.88)0.65 (0.24-1.75)
    P for linear trend (categorical)0.510.88
Sleep duration in 1975, follow-up 1976 to 1996, three-category classification
    ≤6 h1,181230.74 (0.48-1.15)0.85 (0.54-1.34)
    7-8 h (reference)9,0531881.001.00
    ≥9 h1,988310.80 (0.55-1.17)0.69 (0.45-1.06)
    P for linear trend (categorical)0.940.34
Subjects with the same sleep duration in 1975 and 1981, follow-up 1976 to 1996
    ≤6 h476141.03 (0.57-1.84)1.10 (0.59-2.05)
    7-8 h (reference)6,3861251.001.00
    ≥9 h53470.62 (0.29-1.33)0.28 (0.09-0.88)
    P for linear trend (categorical)0.310.03
  • * Adjusted for the effects of age, zygosity, social class, number of children, use of oral contraceptives, body mass index, alcohol use, smoking, and physical activity.