Sleep electroencephalogram spindles are associated with efficient cortical–subcortical connectivity, and intellectual and learning abilities. In the present study, we assessed healthy preschoolers with a twofold aim: (i) to explore associations of non-rapid eye movement S2 spindles with emotional/behavioural characteristics cross-sectionally; and (ii) longitudinally. A total of 43 children who were 5 years old underwent objective sleep electroencephalogram monitoring in their homes. Emotional and behavioural dimensions were assessed by parents and teachers with the Strengths & Difficulties Questionnaire at baseline and at follow-up 1 year later. Non-rapid eye movement S2 spindles were visually scored and compared with Strengths & Difficulties Questionnaire dimensions. High non-rapid eye movement S2 spindle density was associated with less internalizing behaviour, more prosocial behaviour and a low total problem score. In girls, high non-rapid eye movement S2 spindle density was related to low hyperactivity, while in boys it was associated with less internalizing behaviour, more externalizing behaviour and more hyperactivity. Longitudinally, a higher number of non-rapid eye movement S2 spindles at 5 years old predicted fewer peer problems 12 months later. In kindergarten children, high non-rapid eye movement S2 spindle density is associated with observable current and future favourable emotional/behavioural patterns. However, gender differences were also found, as should be taken into account in future studies.