Introduction: the COVID-19 pandemic has brought changes in routines, use of time, behaviors, social relationships and concerns that can compromise children’s sleep, and studies on the subject are essential. Objective: to evaluate the sleep habits of preschool children after the COVID-19 lockdown in Brazil. Methods: this is a cross-sectional study nested in a cohort of live births to assess growth and development up to 1,000 days of age. For this study, data were collected from children at 4 years of age related to biological profile, health status, maternal care, screen time and physical activity, and behavior during the COVID-19 pandemic. The mean scores of the children’s sleep habits (bedtime routine, rhythmicity and affective separation) were analyzed according to the children’s characteristics using the Student’s t-test. Results: sleep routine was the most impaired sleep habit, with lower averages in cases of hospitalization (p = 0.047), mother’s difficulty in caring for the child (p = 0.003) and great concern about COVID-19 during the pandemic (p = 0.003); followed by rhythmicity, which was also worse in the previous situations. In addition, children with more than 60 minutes of recreational screen time (p = 0.002) and without a mask-wearing routine during the pandemic (p = 0.003) had lower average bedtime routines. Health problems at birth (p = 0.001), hospitalization (p = 0.000), special health needs (p = 0.025) and mother’s difficulty in providing care (p = 0.037) negatively interfered with affective separation. Conclusion: children with health problems, excessive screen time and concern about COVID-19 during the pandemic, as well as the difficulty of maternal care, influenced children’s sleep habits. [ABSTRACT FROM AUTHOR]