The goal of this prospective randomised controlled trial is to examine the effects of cognitive behavioural therapy and bright light therapy in youths with unipolar depression and evening chronotype. The main questions it aims to answer are:

1. What is the efficacy of CBT-D and CBT-D plus bright light therapy in reducing depression severity in adolescents with depression and eveningness?
2. What are the effects of CBT-D and CBT-D plus bright light therapy on the subjective and objective sleep and circadian measures, as well as the quality of life, daytime symptoms, and functioning (e.g., sleepiness, fatigue)?

Participants will participate in 8 weekly group sessions of CBT-D intervention based on the well-established CBT elements for treating depression. Concurrently participants will also be asked to wear a portable light device at home for 30 minutes daily for seven weeks, starting from the second week of the group intervention. Participants in the CBT-D only group will receive a placebo light via the device, whereas participants in the CBT-D plus light therapy group will receive the active bright light via the device.

Adolescence is a vulnerable period often linked to the emergence of emotional and behavioural difficulties, particularly major depressive disorder (MDD). It is also associated with a gradual change of circadian rhythm characterised by an intrinsic phase delay with an increased evening preference linked to pubertal development. The optimal treatment for youth depression remains controversial. CBT for depression (CBT-D) is one of the most extensively tested psychological treatments for managing mild to moderate depression. Nonetheless, well-designed randomised controlled trials (RCTs) of CBT-D conducted in young people were limited, and the existing research showed that more than 60% of youth did not respond to psychological treatments. Therefore, improving the current model of care and outcomes in youth with depression is imperative, yet rigorous controlled trials of the treatment for youth depression remain limited. In addition, despite the close interplay between circadian disruption and mood problems, little is known as to whether the adjunctive circadian-focused intervention could potentially enhance the treatment outcome of adolescent depression, especially in those with circadian vulnerability (eveningness).

The study is a randomised controlled trial that examines the effects of cognitive behavioural therapy and bright light therapy in youth with unipolar depression and evening chronotype. Eligible participants will be randomised to one of the following groups: CBT-D plus bright light therapy (CBTD+BLT), CBT for depression only (CBTD-BLT), or waiting list control (WL) using an automated online system. For both treatment groups, the intervention will consist of 8 weekly group sessions (90-min, 4-6 adolescents in each group) of CBT-D delivered in the afternoon/evening after school within a 10-week window. The intervention is structured and based on the well-established CBT elements for treating depression, which combines cognitive and behavioural strategies targeting the common difficulties and problems encountered by depressed adolescents. Participants in both treatment groups will also be introduced to the daily light-based intervention by wearing a portable light device (Re-timers) at home for 30 minutes daily for seven weeks, starting from the second week of the group intervention. Participants in the CBTD-BLT group will receive a placebo light intervention, where they will be exposed to dim red light (50 lux), whereas participants in the CBT-D plus light therapy group will receive the active light treatment. All participants initially randomised to the WL condition will be offered treatment opportunities at the end of the WL period.

The primary aim of the study is to test the efficacy of CBT-D and CBT-D plus bright light therapy in reducing depression severity in adolescents with depression and eveningness when compared with the waiting list control. The secondary aim of the study is to examine the effects of CBT-D and CBT-D plus bright light therapy on the subjective and objective sleep and circadian measures, as well as the measures of quality of life, daytime symptoms and functioning (e.g., sleepiness, fatigue).

Inclusion Criteria:

1. Chinese aged 12-20 years old.
2. Written informed consent of participation into the study is given by the participant and his/her parent or guardian (for those aged under 18).
3. Being able to comply with the study protocol.
4. Having a DSM-5 diagnosis of depressive disorders.
5. Having a score of ≥ 40 on Children's Depression Rating Scale (CDRS-R).
6. Having a score of ≤ 41 on Horne-Östberg Morning-Eveningness Questionnaire (MEQ; classified as evening chronotype).
7. Having a sleep onset time of 11:15pm or later for 12 year olds, 11:30pm or later for 13-14 year olds, and 12:00pm or later for 15-20 years at least 3 nights per week in the past 3 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, neurodevelopmental disorders, organic mental disorders, or intellectual disabilities.
2. Initiation of and change of medication that may interfere with circadian rhythm within past 3 months (e.g., lithium, exogenous melatonin, melatonergic antidepressants).
3. In the opinion of the research clinician, having a clinically significant suicidality (presence of suicidal ideation with a plan or an attempt).
4. Having been enrolled in any other clinical trial investigational products within one month at the entry of the study.
5. Initiation of or change in antidepressant medication within past 3 months.
6. Having been or is currently receiving any structured psychotherapy.
7. With hearing or speech deficit.
8. Night shift worker.
9. Trans-meridian flight in the past 1 month and during intervention.
10. Presence of an eye disease (e.g., retinal blindness, severe cataract, glaucoma).