Despite extensive research on sleep and psychosocial functioning, an important gap within the literature is the lack of inquiry into the direction of effects between these 2 constructs. The purpose of the present 3-year longitudinal study was to examine bidirectional associations between sleep (quality and duration) and 3 indices of psychosocial functioning (intrapersonal adjustment, friendship quality, and academic achievement). We also assessed the role of gender as a possible moderator of the patterns of results. Participants were 942 (71.5% female) emerging adults enrolled at a mid-sized university in southern Ontario, Canada, who ranged in age from 17 to 25 years (M = 19.01 years, SD = 0.90) at the first assessment. Students completed surveys in the winter term for 3 consecutive years, beginning in their first year of university. Survey measures included demographics, sleep quality and duration, intrapersonal adjustment (depressive symptoms, stress, and self-esteem), friendship quality, and academic achievement. Results of path analyses indicated a significant bidirectional association between sleep quality and intrapersonal adjustment. We also found evidence for unidirectional associations, such that better friendship quality and higher academic achievement predicted better sleep quality over time. Overall, psychosocial functioning was more strongly associated with sleep quality relative to sleep duration. Our findings highlight the importance of a longitudinal and holistic approach in understanding the link between sleep and psychosocial functioning among emerging adults at university.