Despite the growing body of research linking sleep problems and social ties, research investigating the direction of effects between these two constructs is lacking. Furthermore, there remains a dearth of research examining the mechanisms that may explain the association between sleep problems and social ties within a longitudinal design. The present 3-year longitudinal study addressed two research questions: (1) Is there a bidirectional association between sleep problems and social ties at university? and (2) Does emotion regulation mediate the association between sleep problems and social ties at university? Participants (N = 942, 71.5 % female; M = 19.01 years at Time 1, SD = 0.90) were university students who completed annual assessments of sleep problems, social ties, and emotion regulation, for three consecutive years. Results of path analysis indicated that the bidirectional association between sleep problems and social ties was statistically significant (controlling for demographics, sleep–wake inconsistency, sleep duration, and alcohol). Analyses of indirect effects indicated that emotion regulation mediated this link, such that better sleep quality (i.e., less sleep problems) led to more effective emotion regulation, which, subsequently, led to more positive social ties. In addition, more positive social ties led to more effective emotion regulation, which, in turn, led to less sleep problems. The findings highlight the critical role that emotional regulation plays in the link between sleep problems and social ties, and emphasize the need for students as well as university administration to pay close attention to both the sleep and social environment of university students.