INTRODUCTION:
Breast cancer (BC) incidence and mortality are increasing globally, including in sub-Saharan Africa (SSA). 1,2 Light exposure at night (LEAN) has been linked to BC risk in high-income countries, but evidence from SSA is lacking.3-5 This study examined the association between LEAN and BC risk in Nigeria.

METHODS:
We conducted a multisite hospital-based case–control study involving 379 histologically confirmed BC cases and 403 cancer-free controls, matched by 5-year age intervals. Participants aged ≥20 years were interviewed in person using a pretested questionnaire. LEAN was assessed as the number of nights per week participants were intentionally or unintentionally exposed to light during bedtime or sleeping hours in the year preceding diagnosis (cases) or interview (controls). Multivariable logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs), adjusting for study site, age, ethnicity, socioeconomic status, reproductive factors, body mass index, physical activity, and night-shift work. Analyses were stratified by age and socioeconomic status.

RESULTS:
After full adjustment, extensive LEAN (≥5 nights/week vs 0 nights/week) was associated with increased BC risk (OR 1.87; 95% CI: 1.09–3.21). Higher BC risk was observed among women aged ≥40 years (OR 2.32; 95% CI: 1.15–4.68), those with tertiary education or higher (OR 2.89; 95% CI: 1.28–6.52), and women earning ≥₦50,000 monthly (OR 3.98; 95% CI: 1.42–11.16). No significant associations were found among younger women or those with lower education or income.

CONCLUSIONS:
Extensive nighttime light exposure was associated with increased BC risk in Nigerian women, particularly among older and higher-socioeconomic-status groups. These novel findings from SSA highlight the potential public health relevance of reducing LEAN during sleeping hours and warrant further investigation.