Although social media use (“SMU”) has been linked to later sleep timing and disturbance, most research has relied on one-time questionnaires and cross-sectional designs. Given that depressed youth endorse more SMU, it is unclear whether these patterns, and SMU’s effect on sleep, emerge during or prior to depression onset. This study used ecological momentary assessment (“EMA”) to assess (i) SMU among youth at high and low familiar risk for depression, and (ii) its relationship to sleep timing, variability, and disturbance. The 83 participating youth (ages 9-14) and their parents completed questionnaires about sleep disturbance at baseline and one year later. The children also completed ecological momentary assessments for nine days. Gender, pubertal status, and depressive symptoms were covaried. This revealed (i) no significant interactions between SMU and risk status with either sleep timing or variability; (ii) no significant interactions between SMU and average bedtime; (iii) significant interactions between SMU and sleep variability; and (iv) significant interactions between risk status and SMU predicting sleep disturbance one year later (based on parent and child reports, controlling for baseline sleep disturbances). High-risk youth were more likely to have prospective increases in sleep disturbance when SMU was higher, but not when SMU was lower. These findings suggest that youth at familiar risk for depression do not exhibit different patterns of SMU relative to their lower-risk peers. SMU was associated with more variability in sleep timing for all youth; however, high-risk may be more likely to develop sleep disturbance when have more SMU. This may indicate a potential pathway through which SMU ay contribute to depression for vulnerable youth.