Sleep-Wake Patterns on Illness Trajectories and Treatment Response in MDD

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)