Control subjects: baseline characteristics for 515 control subjects by quartile level of melatonin (ng/mg of creatinine)
| Q1 | Q2 | Q3 | Q4 | ||
|---|---|---|---|---|---|
| <6.7 | [6.7–12.8) | [12.8–22.2) | ≥22.2 | Ptrend | |
| 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)a | 47.3 (7.7) | 48.3 (6.4) | 48.7 (6.1) | 48.0 (6.8) | 0.36 |
| % White | 78.7 | 74.6 | 82.2 | 89.2 | 0.02 |
| % > High school education | 50.0 | 57.4 | 52.4 | 67.4 | 0.02 |
| % Bilateral oophorectomy | 25.2 | 20.0 | 16.3 | 20.9 | 0.31 |
| % Family history of breast cancer in first-degree relative | 14.2 | 13.8 | 17.0 | 13.2 | 0.71 |
| % Prior breast biopsy | 28.0 | 22.2 | 19.0 | 19.2 | 0.08 |
| % Age of menarche <12 y | 23.6 | 22.3 | 23.3 | 21.1 | 0.69 |
| % Nulliparous | 10.2 | 13.3 | 10.9 | 10.2 | 0.48 |
| % Age at first birth <20 yb | 20.6 | 16.5 | 12.6 | 18.8 | 0.82 |
| % Ever oral contraceptive user | 37.8 | 44.6 | 33.3 | 35.7 | 0.35 |
| % Ever estrogen and progesterone use | 26.8 | 30.8 | 17.8 | 23.3 | 0.17 |
| % Ever smoked | 44.4 | 42.6 | 34.7 | 59.4 | 0.05 |
| % ≤ 6 h of sleep | 34.7 | 41.7 | 26.4 | 30.5 | 0.09 |
| % Insomnia | 29.2 | 28.2 | 29.2 | 23.6 | 0.56 |