The theoretical framework is cognitive-behavioral therapy that includes sleep education, sleep hygiene, sleep restriction, stimulus control, cognitive interventions and relaxation. The intervention has been adapted to adolescents by reducing the amount of psychoeducational information and the interventions related to changing cognitions. It involves six weekly face-to-face individual sessions with a therapist, supported by a participant workbook that contains information, work- sheets, and at-home tasks. Clinical psychologists or graduate clinical psychologists in training delivered the intervention sessions under supervision by the first author. Parents/care-givers were given information about the components, but did not take active part of the intervention.

The Insomnia Severity Index, ISI -adolescent version18 was used before, after at three months follow-up to measure insomnia symptom severity. ISI-a is an adaptation to adolescents of the Insomnia Severity Index32,36. ISI-a is a seven-item measure that yields a quantitative index of sleep impairment and treatment outcome. Total scores range from 0 to 28, with higher scores indicating greater perceived insomnia severity.

Subjective sleep onset latency was measured using a sleep-wake diary (SWD) before, after and at three-months follow-up. Participants filled in the SWD during seven consecutive days/nights at all time points.