Poor sleep quality and inadequate sleep in adolescents are a rising trend globally. The Theory of Planned Behaviour (TPB)-which centres on an individual's attitude toward performing the behaviour, subjective norms and perceived behavioural control-has been applied to examine sleep hygiene behaviours in young adults. We expanded on prior works by using a longitudinal design to examine the effects of TPB factors, together with sleep hygiene knowledge and planning constructs, on sleep hygiene behaviours and on sleep quality and health in a group of Iranian adolescents. A total of 1822 healthy adolescents (mean age = 13.97) from 25 high schools in Qazvin, Iran, completed a self-reported survey at baseline and 6 months later. Structural equation modelling (SEM) was used to delineate the pathway from adolescents' sleep hygiene knowledge, TPB constructs of their behavioural intentions and sleep hygiene behaviours and their sleep quality and self-reported health. The SEM model demonstrated that although behavioural intention, coping planning and action planning predicted the sleep hygiene behaviours positively 6 months later with acceptable model fit [comparative fit index (CFI) = 0.936; Tucker-Lewis index (TLI) = 0.902; root mean square error of approximation (RMSEA) = 0.080; standardized root mean square residual (SRMR) = 0.044], sleep hygiene knowledge did not predict behavioural intentions significantly. Sleep hygiene behaviours were associated with sleep quality and psychiatric wellbeing. Thus, the TPB, combined with coping and action planning, is useful in understanding the sleep hygiene behaviours of adolescents. Health-care providers may want to emphasize TPB constructs and coping and action planning to improve adolescents' sleep hygiene behaviours, rather than rely solely upon increasing adolescents' sleep hygiene knowledge.