Table 2.
Risk of breast cancer associated with indicators of indoor light-at-night among 106,731 study participants: adjusted hazard ratios (HRs) and 95% confidence intervals (95% CI) estimated from Cox proportional hazard models.a
| Indoor light-at-night | Category | No. Casesb | HR (95% CI) | Test for (trend) |
|---|---|---|---|---|
| Uses light | Non-user | 4,869 | 1.0 | |
| Yes | 226 | 1.03 (0.90–1.18) | - | |
| Duration (months) | Non-user | 4,869 | 1.0 | |
| 0–3 | 97 | 1.09 (0.89–1.33) | ||
| 4–6 | 19 | 0.92 (0.59–1.45) | ||
| 7–9 | 11 | 0.79 (0.44–1.43) | ||
| 10+ | 80 | 1.08 (0.87–1.35) | P = 0.62 | |
| Days/week | Non-user | 4,869 | 1.0 | |
| 1–3 | 122 | 1.14 (0.95–1.37) | ||
| 4–5 | 25 | 0.87 (0.59–1.30) | ||
| 6+ | 60 | 0.97 (0.75–1.25) | P = 0.97 | |
| Hours/day | Non-user | 4,869 | 1.0 | |
| 1–2 | 66 | 1.01 (0.79–1.29) | ||
| 3–4 | 43 | 1.07 (0.80–1.45) | ||
| 5–6 | 43 | 1.03 (0.76 to 1.40) | ||
| 7+ | 56 | 1.06 (0.81–1.38) | P = 0.55 | |
| Summary c | Non-user | 4,869 | 1.0 | |
| Light | 45 | 1.17 (0.87–1.57) | ||
| Medium | 109 | 0.99 (0.82–1.20) | ||
| Heavy | 44 | 1.13 (0.84–1.52) | P = 0.53 |
adjusted for: age strata (1-year), race/birthplace, family history of breast cancer, age at menarche, pregnancy history, breast feeding history, physical activity, BMI, alcohol consumption, menopausal status and hormone therapyuse combination, smoking status, and smoking pack years, neighborhood SES and urbanization.
Number of cases does not always sum to total (n=5,095) due to missing/unknown values for light-at-night usage.
Summary : light = uses light 0–3 months and 1–3 days/week and 1–2 hours/night; heavy = uses light 10+ months and 5+ days/week and 7+ hours/day; medium = all other combinations of monthly, weekly, and hourly usage.