Depression and cannabis use (CU) are widespread public health concerns in the US, with a profound and growing impact on young adults and disadvantaged individuals. Depression is a chronic and recurring mental health disorder linked to suicide attempts, mortality, and adverse mental and physical health later in life. Rates of depression in young adults have increased dramatically over the past 20 years. Coinciding with changing trends in depression are equally dramatic changes in CU. Although relieving symptoms of depression is a key motive for CU in young adults using cannabis medically, long term CU can worsen depression over time, leading to increased burden. Critically, limited research has examined the mechanisms underlying the complex and inconsistent link between CU and depression. Two potential mediators of this link are sleep and negative affect. While CU may makes it easier to fall asleep in the short term and can thus improve acute symptoms of depression, long-term CU worsens sleep phenotypes and leads to dysregulated sleep - a known antecedent of major depressive episodes. Similarly, while CU can improve negative affect, especially in clinical samples, longitudinal evidence consistently finds that CU contributes to the development of mood disorders, including depression. Further, despite these known links and despite negative affect being a core feature of mood disorders, research has yet to evaluate the links between CU and depression through negative affect and sleep. This study seeks to fill these knowledge gaps with a prospective, observational study leveraging novel technologies to assess the link between CU and depression both in-the-moment and longitudinally. The specific aims are to examine sleep and negative affect as short and long-term mechanisms linking CU and depression at the day- and person-level. The study will use ecological momentary assessment (EMA) to measure behaviors and wearable biosensors to passively and unobtrusively measure sleep. Young adults (18-34 years; N=65) stratified by frequency of CU and oversampled for low socioeconomic status (income ≤ 200% FPL) will complete 14-day bursts of EMA at baseline, 3, 6, and 9 months. Measures include self-reported and objective sleep phenotypes, CU behaviors, negative affect, depressive symptoms, and inflammatory and endocrine biomarkers of depression. Results will have significant public health impact, inform treatment and policy, and address a critical gap in our understanding of how cannabis and depression are related.