ABSTRACT As one of the most common pregnancy complications, nearly 600,000 women in the United States suffer from postpartum depression annually. Consequences of postpartum depression are far-reaching and include maternal suicide, functional impairment, and impaired development and psychiatric illness in offspring. Despite the high prevalence and significant consequences, under current treatment paradigms less than 16% of women with postpartum depression receive treatment and only 3.2% in total achieve remission. Treatment uptake in this population is influenced by concerns regarding pharmacotherapy during lactation and caregiving demands for a newborn. Novel treatments that remove the significant obstacles to accessing care inherent to the postpartum period, such as non-pharmacological therapies delivered in-home, are urgently needed. Within the Research Domain Criteria (RDoC) domain of arousal/regulatory systems, the circadian timing system and its alignment with the sleep-wake cycle—called the phase angle difference—represents a promising novel treatment target. Morning bright light therapy advances circadian timing, and is an effective antidepressant for seasonal and non-seasonal depression. Importantly, bright light therapy is a non-pharmacological intervention that is administered in-home. However, its efficacy in postpartum depression, and mechanism of action, have not been confirmed. Further, traditional light therapy delivered via light box has limited utility for postpartum women, as the user must remain stationary during the treatment and it may expose the infant to light. Through recent technological innovations, wearable bright light therapy devices (i.e., Re-Timer®) are now available; advantages for postpartum women include freedom of movement during treatment and targeted light exposure limited to the user’s ocular area, so infants are not exposed to the light. The proposed study is a randomized controlled trial of Re-Timer® bright light therapy relative to dim (credible and biologically inactive) placebo light for postpartum depression with the following specific aims: AIM 1: Establish the effect of a wearable morning bright light therapy (n = 60) vs. dim light placebo (n = 60) on clinician-rated postpartum depression symptoms. AIM 2: Establish changes in phase angle difference (relationship between sleep and circadian timing) as a predictor of improvement in postpartum depression. AIM 3: Test the effect of morning bright light therapy on secondary outcomes (self-reported depression and maternal functioning). Treatment adherence will be objectively assessed, and objective assessment of sleep and circadian timing will allow for mechanistic testing. This project will be the first to test the efficacy of wearable bright light therapy for postpartum depression and to establish sleep-circadian alignment as a mechanism of treatment. It will lay the foundation for integration of morning bright light therapy into standard care for postpar...