Age and age-standardized baseline characteristics of the study population at baseline by category of years spent in rotating night shift work.

Data were expressed as mean (SD) or percentage, unless otherwise specified. The number of missing data in NHS I: BMI (n = 4,825); physical activity level (n = 126); diet information (n = 14,612); smoking status (n = 145); menopausal status and hormone use (n = 3,229); sleep duration (n = 8,964); snoring frequency (n = 9,031). The number of missing data in NHS II: BMI (n = 252); physical activity level (n = 393); diet information (n = 19,496); smoking status (n = 473); menopausal status and hormone use (n = 649).aBecause of small sample size in this category, the percentage might not be accurate. In 2001 for NHS II, the corresponding percentage was 95%.