Objectives: Recent evidence indicates that adolescents’ motivation to change sleep-wake patterns is low, despite significant impact of adolescent sleep problems on many areas of daytime functioning. The aim of the present study is to evaluate components of adolescents’ motivation, and subsequent changes in behaviour. Methods: Fifty-six adolescents, aged 13–23 (M = 15.8 ± 2.3 y; 38% m) diagnosed with Delayed Sleep-Wake Phase Disorder (DSWPD) underwent three therapy sessions involving bright light therapy to phase advance sleep patterns. Adolescents were instructed to advance wake-up times by 30-min daily. Motivation ratings of desire, ability, reason, need and commitment to change sleep patterns were taken at baseline. Sleep diaries were taken at the end of treatment session 1, with sequentially earlier wake-up times in 30-min intervals indicating compliance. Results: At the outset of therapy, adolescents indicated strong desire, reasons and need, yet moderate ability and commitment to advance their sleep-wake patterns. Following therapy, sleep-onset times were significantly advanced, total sleep time increased and sleep latency decreased (all p < 0.05). Therapy lasted 6–27 days (M = 13.9 ± 4.5) and clients complied for approximately half the time (between 3 and 15 days; M = 8.8 ± 2.7). Commitment was associated with ability (r = 0.66, p < 0.001) but not desire, reason or need (all p > 0.05). Adolescents’ desire to change (r = 0.30, p = 0.03) and commitment (r = 0.30, p = 0.03) were positively correlated with behaviour change, but their need, ability and reasons were not. A mediation analysis showed that ability and desire were important in predicting behaviour change, by total effects through commitment (ie, indirectly and directly). Conclusion: Our findings suggest that the total effects of ability (ie, confidence) and desire to change are the best predictors of behavioural changes, thus clinicians should focus on these components of the readiness to change model when undertaking treatments with sleep-disordered adolescents.