SummaryInsufficient sleep duration among adolescents is a widespread public health problem. Gaining better insight into social‐cognitive determinants associated with adolescent sleep duration is necessary for developing effective preventive interventions to support healthy sleep. This study aimed to explore whether social‐cognitive determinants regarding sufficient sleep duration were associated with sleep duration, and if these associations were mediated by collective sleep hygiene practices. Furthermore, we examined these associations for social‐cognitive determinants related to not using media before bedtime and doing relaxing activities and considered whether these associations were mediated by specific sleep hygiene practices. Data were collected amongst second‐ and third‐grade adolescents from 10 Dutch high schools. A total of 878 adolescents (mean [SD] age 13.3 [0.71] years) completed data on sleep duration, social‐cognitive determinants of the Theory of Planned Behaviour (i.e., attitude, subjective norms from parents, subjective norms from peers, perceived behavioural control, intention), and sleep hygiene practices. Single‐ and multivariable path models were constructed and mediation by sleep hygiene practices was analysed by Monte Carlo simulation. All social‐cognitive determinants except for subjective norms from peers were associated with longer sleep duration (p < 0.01). Sleep hygiene practices mediated all associations between social‐cognitive determinants and sleep duration (mediation ranging from 16% to 72%). Although some of the significant associations and mediation disappeared in the multivariable model, behavioural arousal was the strongest mediator, but collective sleep hygiene practices and cognitive/emotional arousal also explained parts of the associations. The findings indicate that social‐cognitive factors should not be overlooked when targeting adolescent sleep duration.