Table 3.

Association Between Self-Reported Night-Shift Work and Breast Cancer Risk Among 69,472 Women Without Incident Cancer at the Second Follow-up, Shanghai Women's Health Study^a

	No. in Total Cohort	No. of Cases	Adjusted for Age		Fully Adjustedb
			HR	95% CI	HR	95% CI
Ever working night shifts
Never	51,238	276	1		1
Ever	18,234	73	0.8	0.6, 1.0	0.9	0.7, 1.1
Frequency (no. of night shifts per month)
0	51,238	276	1		1
>0–<8	2,681	8	0.5	0.3, 1.1	0.6	0.3, 1.2
8	10,475	45	0.8	0.6, 1.1	0.9	0.7, 1.3
>8	5,078	20	0.7	0.5, 1.1	0.9	0.5, 1.3
P-trend			0.03		0.29
Duration (years of working night shifts)
0	51,238	276	1		1
>0–≤5	6,055	25	0.8	0.5, 1.2	0.9	0.6, 1.3
>5–≤17	6,459	29	0.8	0.6, 1.2	0.9	0.6, 1.4
>17	5,720	19	0.6	0.4, 1.0	0.8	0.5, 1.2
P-trend			0.03		0.26
Cumulative exposure (total no. of night shifts)
0	51,238	276	1		1
>0–≤576	6,370	27	0.8	0.5, 1.2	0.9	0.6, 1.3
>576–≤1,632	5,924	28	0.9	0.6, 1.3	1.0	0.7, 1.5
>1,632	5,940	18	0.6	0.3, 0.9	0.7	0.4, 1.1
P-trend			0.02		0.17
Age at starting night-shift work, years
Never did shift work	51,238	276	1		1
>30	5,594	18	0.6	0.4, 1.0	0.7	0.5, 1.2
>21–≤30	5,768	25	0.8	0.5, 1.2	0.9	0.6, 1.3
>0–≤21	6,872	30	0.8	0.6, 1.2	0.9	0.6, 1.4

Abbreviations: CI, confidence interval; HR, hazard ratio.

^aFollow-up extended from the second follow-up (2002–2004) to December 2007, death, or the first diagnosis of incident cancer.

^bAdjusted for age, education, family history of breast cancer, number of pregnancies, age at first birth, and occupational physical activity.