Can a night owl pill fix bipolar mood swings?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Capable of giving informed consent * Meet The Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria for bipolar disorder (BD) I or II * Evening chronotype per the Morningness-Eveningness Questionnaire (MEQ) defined by a score of \<42 * At least mild depressive symptoms on the Patient Health Questionnaire (PHQ)-9 defined by a score ≥5 * Psychotropic medications at stable dose for past month * Able to download the MyDataHelps mobile application (app), and open app on participants' own phone * Willing to abstain from alcohol for the duration of the intervention phase * Female participants of childbearing potential (i.e., patients are not permanently sterilized (hysterectomy, bilateral salpingectomy, and bilateral oophorectomy) or postmenopausal (12 months with no menses without an alternative medical cause) by report) must agree to use a reliable method of contraception from the screening visit until 4 weeks after the study has completed. Exclusion Criteria: * Current diagnosis of, or high risk for, a sleep disorder other than DSPD per interview and medical record review (when available) including: * Insomnia per DSM-5 * Sleep-disordered breathing per Snoring, tiredness, observed apnea, blood pressure, body mass index, age, neck circumference, and gender (STOP-BANG) * Restless leg syndrome per sleep interview * Narcolepsy * Suspicion of vasomotor symptoms impacting sleep per interview for women that may be perimenopausal or postmenopausal. * Risk of current mania (per Young Mania Rating Scale (YMRS) score \> 19). * Suicidal or at high risk for suicide per Columbia Suicide Severity Rating Scale (C-SSRS) guidelines (i.e., presence of any suicidal behavior-suicide attempt, interrupted attempt, abort attempt, or preparatory behavior-in the past 3 months; and/or current active suicidal ideation with any intent), or as determined by the principal investigators. * Presence of cardiac implantable electronic device, such as defibrillator or pacemaker. * Presence of chronic psychiatric conditions which may directly influence sleep per interview and medical record review (when available), including: * Current illicit drug use per the Drug Use Disorders Identification Test (DUDIT) defined by a score of ≥ 25 * Current alcohol or drug abuse per the Alcohol Use Disorder Identification Test (AUDIT) defined by a score of ≥ 16 and DUDIT * Currently experiencing psychosis * Presence of unstable chronic medical condition which may directly influence sleep: * Chronic pain * Thyroid conditions * Current or history of medical conditions which may be affected by melatonin per self-report and medical record review (when available), such as: * Hypertension or hypotension * Diabetes Type 1 or Type 2 * Clotting/bleeding disorders * Epilepsy/seizures * Autoimmune disorders * Conditions requiring immunosuppressive management such as transplant * Per self-report or medical record review (when available), current use of medications which may have interactions with melatonin (see protocol for more details). * Current use of medications that may interfere with the measurement of melatonin (non-steroidal anti-inflammatory drugs if used daily, and beta-blockers), per self-report and medical record review (when available). * Self-report use of melatonin in the past month. * Hypersensitivity to melatonin or any other component of the melatonin or placebo product. * Pregnancy (as determined by dipstick urinary pregnancy test at screening for women of child-bearing potential) or self-report of breastfeeding and/or plan to become pregnant in the next 3 months. * Self-report of routine night shift work. * Self-report of past month travel or planned travel during the study across more than one time zone.