Nocturnal Pain and Sleep Disorders in Women with Breast Cancer: A Cross-Sectional Study with Multidimensional Assessment in a Brazilian Cohort

Background and Objectives: Breast cancer is the most common malignancy among women in Brazil (41.89/100,000 women, 2023–2025) and is associated with high psychosocial morbidity. Sleep disturbances are frequent in this population and pain is a known contributor, but the time of day when pain occurs has rarely been considered, particularly in Latin American cohorts. This study aimed to assess the prevalence of self-reported nocturnal pain and its associations with insomnia, sleep quality, daytime sleepiness, obstructive sleep apnea (OSA) risk, anxiety, and depression in women with breast cancer. Materials and Methods: This is an analytical cross-sectional study with 211 patients under oncological outpatient follow-up at three referral hospitals in Greater São Paulo, Brazil (CEP-FMABC approval No. 7,920,023). Five validated instruments were applied: Insomnia Severity Index (ISI), Pittsburgh Sleep Quality Index (PSQI), Epworth Sleepiness Scale (ESS), STOP-Bang, and Hospital Anxiety and Depression Scale (HADS). Comparisons were performed using t-tests and chi-square tests. Adjusted odds ratios (aORs) were estimated by multiple logistic regression adjusted for anxiety, depression, age, body mass index (BMI), and menopausal status. Sensitivity analyses added medication burden, comorbidities, and oncological treatment history, and used Firth penalized regression. Results: Nocturnal pain prevalence was 29.9% (n = 63; 95% confidence interval [CI]: 23.8–36.5). The pain group showed significantly worse ISI, PSQI, HADS-Anxiety, and HADS-Depression scores. Nocturnal pain was associated with poor sleep quality (aOR = 3.54; 95% CI: 1.49–8.45; p = 0.004) and clinical insomnia (aOR = 2.16; 95% CI: 1.08–4.33; p = 0.029). Point estimates were stable in sensitivity analyses (aOR 3.40–3.67 for poor sleep quality and 1.92–2.16 for insomnia in conventional logistic models), although the confidence interval for insomnia included unity after additional adjustment for comorbidities. Conclusions: Self-reported nocturnal pain was associated with poor sleep quality and insomnia in women with breast cancer, beyond anxiety and depression symptoms. Given the cross-sectional design, these associations do not establish causality. Asking about pain at night is simple and may help identify patients who need a sleep assessment.