Background. Children’s sleep problems are associated with temperament. One of the dimensions of temperament – higher emotional reactivity – is defined as a risk factor for children’s emotional, behavioral, and sleep problems. Screen-based media use is a very common phenomenon among children that relates to sleep problems. Still there is a gap of research explaining the interactions between children’s sleep problems, temperament, and parental discipline (sleep regime and rules for screen-based media use). The aim of this study is to evaluate the relations between preschool children’s sleep problems and parents’ discipline (sleep regime and rules for screen-based media use), and the role of children’s emotional reactivity. Methods. This research is a part of the longitudinal study “Electronic Media Use and Young Children’s Health” conducted in the year 2017–2018 and funded by the Research Council of Lithuanian (agreement no. GER-006/2017). Participants are 876 children aged 2 to 5 years old and their parents. Children’s sleep problems and emotional reactivity were assessed using the Child Behavior Checklist (CBCL/1½-5). Information about rules for screen-based media use and sleep regime was obtained using the parent-report questionnaire. Results. Sleep problems are related to emotional reactivity, sleep regime, and rules for screen media use. Children without regular sleep regime and without constant rules for screen-based media use have higher sleep problems and emotional reactivity. The results of the regression analysis show that emotional reactivity, together with sleep regime and rules for screen-based media use, significantly explain one-fifth to one-third of children’s sleep problems at different ages of the preschool period. However, the prognostic value of emotional reactivity and parental discipline varies according to a child’s age, as they are significant predictors of sleep problems among two, three and four-year-olds, but no longer explain sleep problems of five-year-olds. The path analysis confirmed that emotional reactivity, directly and through mediating variables, e.g., parental reported child’s sleep regime and rules for screen-based media use, is significantly associated with children’s sleep problems. Conclusions. Emotional reactivity should be considered as a significant risk factor in the relation between children’s sleep problems, sleep regime and parental applied rules for screen-based media use. These results are important while identifying children at higher risk for sleep problems. The results also support that parental discipline, such as sleep regime and rules for screen-based media use, are significant for preventing sleep problems in children with higher emotional reactivity.