Insomnia and attention deficit hyperactivity disorder (ADHD) are highly comorbid conditions with an intricate, bidirectional relationship. In particular, insomnia and ADHD often co-occur, with 22.9-34.6% of adolescents with insomnia having clinically significant ADHD, and 22-33.5% of adolescents with ADHD experiencing comorbid insomnia symptoms. Insomnia is not only associated with increased ADHD severity, but also linked to an increased risk for other mental health problems and poorer treatment response in young people with ADHD. Behavioural sleep interventions have shown promise in improving sleep in young children with comorbid ADHD and insomnia. Nonetheless, when treating adolescent insomnia, especially those with ADHD, there is a need to consider the unique sleep and circadian features as well as psychosocial factors associated with not only ADHD but also adolescence, a challenging developmental stage. To date, there is a paucity of research to test the efficacy of cognitive behavioural therapy for insomnia (CBT-I) in adolescents with comorbid ADHD and insomnia. To address the limitations in the existing literature, this study aims to conduct a randomised controlled trial (RCT) to examine the effects of CBT-I relative to a usual care (UC) condition in adolescents with ADHD, on improving sleep and ADHD symptoms, and other clinical and daytime symptoms, as well as overall functional improvement.

A parallel-group, randomised controlled trial will be conducted in adolescents with comorbid ADHD and insomnia. Eligible participants will be randomised to either CBT-I + UC or UC condition. Assessments will be conducted at baseline, one-week post-treatment, and post-treatment 3-month. A random subset of the sample (estimated 15 participants per group) will be assessed for sleep objectively using actigraphy for 7 consecutive days at baseline and one-week post-treatment. Participants in the control group will be offered CBT-I after 3-month follow-up. During the study period, all the participants will continue their regular clinical follow-ups with their attending psychiatrist for the prescription of psychotropic medication(s), if needed, for managing their ADHD and mental health condition. Prescription of any psychotropic medications of each participant during the trial will be reviewed and documented at baseline and at each follow-up.

Inclusion Criteria:

1. Cantonese-speaking adolescent aged 12-18 years old;
2. A diagnosis of ADHD as confirmed by the Diagnostic Interview Schedule for Children-version-IV (DISC-IV);
3. Having a DSM-5 diagnosis of insomnia disorder and with a score on Insomnia Severity Index (ISI) ≥ 9 (suggested cut-off for adolescents);
4. Written informed consent of participation into the study is given by the participant and his/her parent or guardian (for those aged under 18);
5. Being able to comply with the study protocol;
6. Those who are not on ADHD medication or have been stabilized on psychostimulant or nonstimulant ADHD medications (maintaining the optimal medication dosage for at least 6 months).

Exclusion Criteria:

1. A current diagnosis of substance abuse or dependence; a current or past history of manic or hypomanic episode, schizophrenia spectrum disorders, autism spectrum disorder, organic mental disorders, or intellectual disabilities;
2. Having a prominent medical condition known to interfere with sleep continuity and quality (e.g., severe eczema, gastro-oesophageal reflux disease);
3. Having a clinically diagnosed sleep disorder that may potentially contribute to a disruption in sleep continuity and quality, such as narcolepsy, sleep-disordered breathing, and restless leg syndrome;
4. Concurrent, regular use of psychotropic medications(s) known to affect sleep continuity and quality (e.g., hypnotics, steroids), except for the use of psychostimulant medication for ADHD;
5. Receiving ongoing psychological treatment for sleep problems;
6. With hearing or speech deficit;
7. In the opinion of the research clinician, having a clinically significant suicidality (e.g., with suicidal ideation with a plan or a suicide attempt in the recent one month), and/or endorsing "nearly everyday" for item 9 (suicidal thoughts) in the Patient Health Questionnaire - 9 (PHQ-9).