Objective: This study aimed to explore the stability and bidirectional effects between sleep problems and effortful control (EC) during the preschool period. Methods: A short-term longitudinal study was conducted using a sample of 202 preschoolers (Time 1: 3-to 5-years old, Time 2: 4- to 6-years old). Sleep problems were assessed using the Korean version of the Children’s Sleep Habits Questionnaire (K-CSHQ), and EC was assessed using the Children’s Behavior Questionnaire Form (CBQ). The mother of the child responded to all items. Data were analyzed using an autoregressive cross-lagged model. Results: Scores for the three aspects of sleep problems decreased while levels of EC increased from Time 1 (T1) to Time 2 (T2). The autoregressive cross-lagged model established that sleep problems exhibited moderate rank-order stability, while EC exhibited a high level of stability over the two time points. Sleep duration/onset delay at T1 had an impact on EC at T2. Additionally, EC at T1 was a significant predictor of sleep behavior problems at T2. Conclusions: The findings indicate that sleep problems during the preschool period tend to persist over time, albeit with moderate stability coefficients, implying a potential for change. Given that persistent sleep problems during the preschool period can have adverse effects on multiple domains of development, the present study underscores the significance of this period as a critical time for implementing preventive or intervention strategies aimed at alleviating sleep problems.