© 2020, © 2020 Taylor & Francis Group, LLC. Objective: This systematic review aimed to summarize correlates of sleep duration in children under 5 years of age. Methods: Six electronic databases (PubMed, PsycINFO, MEDLINE, CINAHL, SPORTDiscus, and Scopus) were searched from inception to May 2019. Observational studies and intervention studies reporting cross-sectional results from baseline data were considered. Data were extracted using a predesigned form and potential correlates were categorized following a sociological framework. Results: One-hundred and sixteen studies, representing 329,166 children, met the inclusion criteria, with a high risk of bias in 62 included studies. A total of 83 correlates of sleep duration were identified. Among the associations studied four or more times, correlates of nap duration were child’s age and nighttime sleep onset/bedtime; correlates of nighttime sleep duration were household income, parent marital status, parental adiposity level, nighttime sleep duration at younger age, nighttime sleep onset/bedtime, nighttime sleep wakeup time, and frequency of current bedtime routine; correlate of total sleep duration was screen time. Conclusions: Young children from low-income households, single families, or having overweight parents may be at risk for short sleep duration. Promoting healthy sleep duration from an early age appears essential. Effective practices may include encouraging an earlier bedtime, limiting screen time, and establishing a regular bedtime routine. The absence of consistent evidence in the psychological, cognitive, and emotional domain as well as the physical environmental domain warrants further research.