Table 1.

Nested case–control study: baseline characteristics for 258 cases and 515 control subjects

Cases, n = 258Controls, n = 515P
Mean urinary melatonin (ng/mg of creatinine; SD)16.3 (11.9)16.1 (12.9)0.80
Mean urinary melatonin (ng; SD)11.6 (11.1)12.3 (12.5)0.43
Mean creatinine (mg; SD)0.79 (.46)0.76 (0.45)0.48
Mean age at enrollment (y; SD)63.6 (7.2)63.5 (7.1)0.82
Mean body mass index (kg/m2; SD)28.5 (6.1)27.5 (6.1)0.04
Mean total energy expenditure from recreational activity (MET h/wk; SD)11.9 (12.8)12.9 (14.6)0.37
Mean alcohol intake (servings/wk; SD)2.4 (6.1)2.1 (4.6)0.41
Mean age at menopausea (y; SD)48.8 (6.6)48.1 (6.8)0.16
% White81.881.20.87
% >High school education65.957.30.002
% Bilateral oophorectomy12.420.60.02
% Family history of breast cancer in first-degree relative22.114.60.02
% Prior breast biopsy33.122.00.001
% Age of menarche <12 y23.322.50.46
% Nulliparous14.311.10.55
% Age at first birth <20 yb21.117.10.38
% Ever oral contraceptive users35.337.90.48
% Ever estrogen and progesterone use27.524.70.39
% Ever smoked51.654.00.33
% ≤ 6 hours versus 9+ hours of sleep33.7 vs. 5.433.0 vs. 5.40.93
% Insomnia29.728.60.70
  • aRestricted to 726 women with natural menopause.

  • bRestricted to 608 parous women.