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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 h 44 0.73 (0.10-5.32) 0.88 (0.11-6.91) 
    5 h 183 0.81 (0.32-2.03) 0.91 (0.36-2.32) 
    6 h 954 17 0.66 (0.40-1.11) 0.74 (0.44-1.26) 
    7 h 3,473 68 0.84 (0.63-1.13) 0.81 (0.60-1.10) 
    8 h (reference) 5,580 120 1.00 1.00 
    9 h 1,650 25 0.73 (0.48-1.13) 0.64 (0.40-1.02) 
    ≥10 h 338 0.82 (0.36-1.88) 0.65 (0.24-1.75) 
    P for linear trend (categorical)   0.51 0.88 
Sleep duration in 1975, follow-up 1976 to 1996, three-category classification     
    ≤6 h 1,181 23 0.74 (0.48-1.15) 0.85 (0.54-1.34) 
    7-8 h (reference) 9,053 188 1.00 1.00 
    ≥9 h 1,988 31 0.80 (0.55-1.17) 0.69 (0.45-1.06) 
    P for linear trend (categorical)   0.94 0.34 
Subjects with the same sleep duration in 1975 and 1981, follow-up 1976 to 1996     
    ≤6 h 476 14 1.03 (0.57-1.84) 1.10 (0.59-2.05) 
    7-8 h (reference) 6,386 125 1.00 1.00 
    ≥9 h 534 0.62 (0.29-1.33) 0.28 (0.09-0.88) 
    P for linear trend (categorical)   0.31 0.03 
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 h 44 0.73 (0.10-5.32) 0.88 (0.11-6.91) 
    5 h 183 0.81 (0.32-2.03) 0.91 (0.36-2.32) 
    6 h 954 17 0.66 (0.40-1.11) 0.74 (0.44-1.26) 
    7 h 3,473 68 0.84 (0.63-1.13) 0.81 (0.60-1.10) 
    8 h (reference) 5,580 120 1.00 1.00 
    9 h 1,650 25 0.73 (0.48-1.13) 0.64 (0.40-1.02) 
    ≥10 h 338 0.82 (0.36-1.88) 0.65 (0.24-1.75) 
    P for linear trend (categorical)   0.51 0.88 
Sleep duration in 1975, follow-up 1976 to 1996, three-category classification     
    ≤6 h 1,181 23 0.74 (0.48-1.15) 0.85 (0.54-1.34) 
    7-8 h (reference) 9,053 188 1.00 1.00 
    ≥9 h 1,988 31 0.80 (0.55-1.17) 0.69 (0.45-1.06) 
    P for linear trend (categorical)   0.94 0.34 
Subjects with the same sleep duration in 1975 and 1981, follow-up 1976 to 1996     
    ≤6 h 476 14 1.03 (0.57-1.84) 1.10 (0.59-2.05) 
    7-8 h (reference) 6,386 125 1.00 1.00 
    ≥9 h 534 0.62 (0.29-1.33) 0.28 (0.09-0.88) 
    P for linear trend (categorical)   0.31 0.03 
*

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.

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