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 established. This administrative supplement is proposed for a parent a randomized controlled trial of wearable bright light therapy relative to dimmed light placebo for postpartum depression. Inspired by emerging evidence that implicates light sensitivity as a novel treatment mechanism for light therapy in seasonal affective disorder, we now have a unique opportunity to leverage ongoing data collection by testing light sensitivity as a treatment mechanism for wearable morning light therapy in postpartum depression. The aims of the administrative supplement are as follows: AIM 1: Compare light sensitivity pre- to post-treatment between women with postpartum depression assigned to bright light therapy (n=15) and dimmed light placebo (n=15). AIM 2: To evaluate associations between light sensitivity and depression symptoms at pre-treatment, as well as changes in light sensitivity and changes in depression symptoms from pre- to post-treatment. This project will be the first to test preliminary effects of wearable bright light therapy on light sensitivity in postpartum depression, associations between light sensitivity and postpartum depression symptoms, and whether changes in light sensitivity are associated with changes in postpartum depression symptoms. It will lay the foundation for understanding novel treatment mechanisms of light therapy, an important step towards personalized medicine approaches for postpartum depression.