**Background:** Sleep is essential for physiological regulation, including hormone release, cardiovascular function, and glucose control. Poor sleep quality, especially when chronic, can impair immune function and antibody production, potentially increasing vulnerability to infectious diseases such as COVID-19. The COVID-19 pandemic and associated containment measures (social restrictions, reduced physical activity, changes in diet, and decreased sun exposure) may have adversely affected sleep quality. Brazil was among the five countries most affected by COVID-19 at the time of the study.
**Methods:** This cross-sectional, population-based household serological survey was part of the COVID-Inconfidentes project in the Iron Quadrangle region of Minas Gerais, Brazil. Data were collected on weekends between October and December 2020. A three-stage conglomerate sampling design was used: census sectors were randomly selected without replacement, households were selected via systematic sampling, and one resident per household was randomly chosen. The final sample comprised 1,762 adults. Sample weights were calculated and calibrated to compensate for non-response. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI), with a global score > 5 indicating poor sleep quality. Data on sociodemographic variables (sex, age, marital/living status, education, income, employment), health conditions (chronic diseases, chronic pain, anxiety, depression, self-rated health), health-related behaviors (smoking, alcohol, physical activity), nutritional status (BMI, weight change ≥5% during the pandemic), sun exposure (<30 vs. ≥30 minutes/day), vitamin D status (sufficient vs. insufficient based on sun exposure and supplementation), and pandemic characteristics (COVID-19 symptoms, serostatus, social restriction, risk group in family, pandemic period) were collected via face-to-face structured interviews. Statistical analyses used Poisson regression with robust variance, accounting for the complex survey design. Variables with P < 0.2 in univariate analysis were entered into a multivariate model with stepwise backward elimination, controlling for pandemic period. Collinearity was assessed; anxiety and self-rated health were collinear, and anxiety was retained.
**Key Results:** The mean PSQI score was 6.32 (95% CI: 6.03–6.62), and 52.5% of participants had poor sleep quality. The most affected PSQI subdomains were sleep latency (45.8% abnormal), sleep disturbance (36.8%), and sleep efficiency (20.1%). Women had higher prevalence of abnormal scores in subjective sleep quality, sleep efficiency, and use of sleep medications (P < 0.05). Sleep medication use increased with age, while daytime dysfunction was higher in younger adults (P < 0.05). In the multivariate analysis, the following factors were significantly associated with poor sleep quality: living alone (PR = 1.34; 95% CI: 1.04–1.73; P = 0.026), anxiety disorder (PR = 1.32; 95% CI: 1.08–1.62; P = 0.008), weight loss ≥5% (PR = 1.21; 95% CI: 1.02–1.44; P = 0.028), weight gain ≥5% (PR = 1.27; 95% CI: 1.03–1.55; P = 0.026), vitamin D insufficiency (PR = 1.16; 95% CI: 1.01–1.35; P = 0.043), and presence of COVID-19 symptoms (PR = 1.29; 95% CI: 1.10–1.52; P = 0.003). Interaction analyses showed that the co-occurrence of two factors increased the prevalence ratio further, with the highest PRs observed for concurrent COVID-19 symptoms and weight loss (PR = 1.72; 95% CI: 1.38–2.15) and vitamin D deficiency with weight gain (PR = 1.67; 95% CI: 1.19–1.91). Only the combination of weight loss and vitamin D deficiency was not significant (PR = 1.09; 95% CI: 0.73–1.62).
**Clinical Implications:** This study demonstrates that poor sleep quality was highly prevalent (52.5%) among Brazilian adults during the COVID-19 pandemic, exceeding rates reported in pre-pandemic studies and early-pandemic systematic reviews (~36% in the general population). The identified associations—particularly vitamin D deficiency and weight changes—are potentially modifiable factors that could be targeted in public health interventions. Vitamin D deficiency may affect sleep through its role in melatonin synthesis and regulation of the sleep-wake cycle via vitamin D receptors in brain areas. Weight changes, whether loss or gain, may reflect pandemic-related stress, food insecurity, or lifestyle disruptions that in turn affect sleep. The strong association with anxiety and living alone underscores the mental health toll of the pandemic and social isolation. The finding that co-occurring risk factors compound the probability of poor sleep suggests that vulnerable individuals with multiple risk factors require prioritized support. Limitations include the cross-sectional design (precluding causal inference), reliance on self-reported weight and height, and the inability to assess pre-pandemic sleep quality. Strengths include the representative household-based sampling, face-to-face interviews during the pandemic, and rigorous statistical methods accounting for complex survey design.