Introduction: One of the most distressing factors for patients in the intensive care unit (ICU) is their inability to sleep. Various factors, including patient and environmental factors, can affect patients' sleep quality. This study aimed to evaluate sleep quality among patients admitted to the ICU and investigate the environmental and non-environmental factors that affect sleep quality in the ICU. Method: A multicentre descriptive cross-sectional study was conducted on 267 patients hospitalized in the intensive care units of six hospitals in the Czech Republic from February to April 2023. On the day of patients' discharge, factors affecting their sleep were determined according to a modified Freedman questionnaire. The data were analysed using descriptive statistics, an ANOVA model, and the multivariate regression method. Results: The total quality of sleep in the ICU was significantly reduced compared to at home (5.16 vs. 6.99, p < 0.001). Men reported significantly higher sleep quality than women (5.26 vs. 4.80, p < 0.001), and people over 60 slept worse (5.99 vs. 6.17, p < 0.001). It was confirmed that sleep quality improved during hospitalization (4.24 vs. 5.20 vs. 5.66, p < 0.001). Among the five most important factors that disturbed sleep were medication administration (0.478), pain (0.370), noise (0.349), light (0.329), and nursing activities (0.299), evaluated according to component loading. Conclusion: Multifactorial causes, including environmental and nonenvironmental factors, can disrupt ICU patients' sleep. Interventions and strategies should be developed to minimize sleep disruption and improve the quality of care provided to ICU patients.