tDCS: Sleep to Mood in Depression (S2M-D)

Inclusion Criteria: 1. Ability to read and understand study materials in English, as demonstrated by a standard score \>85 on the WRAT-5 Reading Recognition Subtest 2. MADRS-S ≥18 during screening 3. Confirmed diagnosis of MDD, based on clinician interview 4. If taking antidepressant medication, the dose must be stable for at least 30 days prior to pre-screening. 5. Confirmed access to a local provider that has primary responsibility for the subjects' clinical care, and who is available for contact in case of increased subject risk due to depressive symptoms OR confirmed acknowledgement and identification of local resources. 6. Able to provide informed consent and comply with study procedures. 7. Access to a quiet space suitable for home-based tDCS stimulation sessions. Exclusion Criteria: 1. Current major medical condition that, in the opinion of the study clinician, would interfere with study participation, safety, or interpretation of study outcomes. 2. Current or past bipolar disorder, psychotic disorder, or current (past 6 months) alcohol or substance use disorder, as determined by clinician assessment at screening. 3. Clinically significant suicidal ideation or behavior, based on clinician assessment and the Columbia-Suicide Severity Rating Scale (C-SSRS). 4. Use of prescription or over-the-counter sleep-promoting medications or initiation of behavioral interventions specifically targeting sleep in the past 30 days from pre-screening. 5. History of seizure disorder or epilepsy. 6. Presence of an active implanted medical device above the neck (e.g., deep brain stimulator, cochlear implant) or non-MRI-compatible metallic implants in the head or neck region. 7. Current use of prescription or over-the-counter sleep medications, or active participation in any behavioral or pharmacologic treatment specifically targeting sleep. 8. Pregnancy and Breastfeeding: Currently pregnant, planning pregnancy during the study period, or breastfeeding o If able to become pregnant, participants will be required to use medically accepted birth control methods, including hormonal methods (e.g., pills, patches, implants), barrier methods (e.g., condoms or diaphragm with spermicide), intrauterine device (IUD), or abstinence. Participants will be advised not to become pregnant during the study.