The proposed study will include a longitudinal ecological study (Study 1) and a randomised controlled trial (Study 2). The aims will be to (1) identify the sleep-wake profiles in individuals with MDD through clustering; (2) examine the associations between sleep-wake features/profiles and the prognosis of MDD; and (3) investigate the anti-depressant effects of sleep- and circadian-targeted intervention in those with MDD and whether sleep-wake features/profiles may moderate the treatment outcomes. 70 depressed participants from Study 1 will be randomized to Study 2 for the Transdiagnostic Intervention for Sleep and Circadian Dysfunction (TranS-C) group and the care-as-usual (CAU) group in a 1:1 allocation ratio. The study period will be 8 weeks. Major assessments at baseline and immediate post-treatment will be managed by an independent assessor (a research assistant, RA) who is blind to the group allocation. The proposed trial and the nested pilot study will follow the CONSORT and STROBE guidelines, respectively. The intervention will be provided free of charge.The current registration is only for Study 2 of the current research project.

Inclusion Criteria for the MDD group:

1. Adults aged 18-65 years living in Hong Kong
2. Being fluent and literate in Cantonese
3. A Patient Health Questionnaire-9 (PHQ-9) score of at least 10, indicating a moderate level of depression.
4. Meeting the DSM diagnostic criteria for current MDD according to the Mini International Neuropsychiatric Interview (MINI)
5. Experiencing ≥ 1 sleep or circadian problem in the past 3 months according to the sleep and circadian problems checklist (i.e., time needed to fall asleep ≥ 30 minutes, wake after sleep onset ≥ 30 minutes, < 6-hour sleep per night or ≥ 9-hour sleep per night per 24-hour period, and falling asleep after 2:00 AM ≥ 3 nights per week; and variability in the sleep-wake schedule ≥ 2.78 hours within a week)
6. Access to an Internet-enabled mobile device (iOS or Android operating system)
7. Willingness to provide informed consent and comply with the trial protocol.

Exclusion Criteria for the MDD group:

1. Severe medical or neurocognitive disorder(s) that make participation unsuitable based on the team's clinical experience
2. Presence of other psychiatric disorders as assessed by MINI
3. A history of electroconvulsive therapy (ECT)
4. Current involvement in any psychological treatment programme that targets depression and/or sleep and circadian problems
5. A change in the class or dose of any prescribed psychotropic drugs within 6 weeks before the baseline assessment
6. Shift work, pregnancy, or work, family, or other commitments that interfere with regular sleep-wake patterns
7. Presence of other untreated sleep disorders, including narcolepsy, obstructive sleep apnoea (OSA), and restless leg syndrome (RLS)/periodic leg movement disorder (PLMD) based on the cut-off scores of SLEEP-50 (narcolepsy ≥ 7; OSA ≥ 15; RLS/PLMD ≥ 7)