Figure 1.

Relative risk of breast cancer in Million Women Study participants who worked night shifts by selected characteristics. *Relative to never night shift workers, stratified by region and with attained age as the underlying time variable and adjustment for socioeconomic status, parity and age at first birth, body mass index, alcohol intake, strenuous physical activity, family history of breast cancer, age at menarche, oral contraceptive use, smoking, living with a partner, and use of menopausal hormone therapy. Relative risks (RRs) are represented by squares (with their 95% confidence intervals [CIs] as lines), each with area inversely proportional to the variance of the log RR, thereby indicating the amount of statistical information for that particular RR.