Table 2.

Control subjects: baseline characteristics for 515 control subjects by quartile level of melatonin (ng/mg of creatinine)

Q1Q2Q3Q4
<6.7[6.7–12.8)[12.8–22.2)≥22.2Ptrend
Mean age at enrollment (y; SD)63.4 (7.4)63.9 (7.5)63.8 (6.3)62.7 (7.3)0.43
Mean body mass index (kg/m2; SD)28.8 (5.7)27.6 (6.0)28.0 (6.9)25.9 (5.2)≤.01
Mean total energy expenditure from recreational activity (MET h/wk; SD)10.9 (13.5)14.8 (17.1)11.5 (13.2)14.4 (13.9)0.22
Mean alcohol intake (servings/wk; SD)2.4 (5.4)1.7 (3.4)2.0 (5.1)2.2 (4.2)0.18
Mean age at menopause (y; SD)a47.3 (7.7)48.3 (6.4)48.7 (6.1)48.0 (6.8)0.36
% White78.774.682.289.20.02
% > High school education50.057.452.467.40.02
% Bilateral oophorectomy25.220.016.320.90.31
% Family history of breast cancer in first-degree relative14.213.817.013.20.71
% Prior breast biopsy28.022.219.019.20.08
% Age of menarche <12 y23.622.323.321.10.69
% Nulliparous10.213.310.910.20.48
% Age at first birth <20 yb20.616.512.618.80.82
% Ever oral contraceptive user37.844.633.335.70.35
% Ever estrogen and progesterone use26.830.817.823.30.17
% Ever smoked44.442.634.759.40.05
% ≤ 6 h of sleep34.741.726.430.50.09
% Insomnia29.228.229.223.60.56
  • aRestricted to 726 women with natural menopause.

  • bRestricted to 608 parous women.