Inclusion Criteria:
1. Aged 12-18 years at the time of screening.
2. Current Major Depressive Disorder (MDD) documented in the electronic medical record by a licensed clinician; diagnosis verified using the Mental Health History - Parent Form.
3. Lives with a parent/legal guardian at least 50% of the time, confirmed on the Demographics Form.
4. Able and willing to provide assent, with a parent/legal guardian able and willing to provide informed consent.
5. Willingness to comply with all study procedures, including wearing a Fitbit Charge 5 for 7 nights, syncing the device through a compatible smartphone, and completing all required assessments (DERS-36, PHQ-9, anthropometrics, Youth Activity Profile, caffeine logs, PACHIQ-R).
6. Primary caregiver willing to participate, including completion of the PACHIQ-R (Caregiver).
7. If taking antidepressant medication, must be on a stable dose for at least 4 weeks before screening, verified on the Self-Administered Comorbidity Questionnaire.
8. English-speaking adolescent and caregiver, as all measures used in the study are validated in English.
9. Available for two in-person study visits within the study period.
Exclusion Criteria
1. Medical conditions affecting autonomic function or HRV (arrhythmia, congenital heart disease, cardiomyopathy, Type 1 or Type 2 diabetes, thyroid disorders, or moderate-to-severe sleep apnea), as reported on the Self-Administered Comorbidity Questionnaire.
2. Use of medications known to affect heart rate or HRV, including beta-blockers, calcium channel blockers, antiarrhythmics, tricyclic antidepressants, stimulants, or antipsychotics, as documented on the NIMH Concomitant Medication Log.
3. Active substance use, including nicotine/tobacco, alcohol, marijuana, recreational drugs, or energy drinks, as assessed by the Drug Use Screening Inventory.
4. Habitual high caffeine intake (\>200 mg/day, equivalent to \>4 cups of coffee or \>2 cans of caffeinated soda per day), as assessed by a caffeine screening question during eligibility screening.
5. Psychotic disorders or active psychotic symptoms, as reported on the Mental Health History Questionnaire (Parent) and the Self-Administered Comorbidity Questionnaire.
6. Acute suicidal ideation requiring immediate intervention, defined as endorsement of agree and strongly agree on items 14-16 of the Concise Health Risk Tracking Self-Report (CHRT-SR), indicating active suicide risk.
7. Severe cognitive or intellectual impairment interfering with study participation, determined during eligibility screening based on parent report on the Mental Health History Questionnaire and observed inability to complete screening or consent procedures.
8. Current eating disorder diagnosis (anorexia nervosa, bulimia nervosa, or binge eating disorder), as reported on the Mental Health History Questionnaire.
9. Concurrent participation in another study involving wearable devices or daily monitoring, as reported on the Eligibility Screening Form.
10. Skin conditions or sensitivities prevent continuous wear of a wrist-worn device reported on the Eligibility Screening Form.
