PROJECT SUMMARY/ABSTRACT The transition to parenthood is a period of profound transformation and often vulnerability in new parents. Though most of the research on this transition has focused on mothers, new fathers must also adapt to parenthood. Many parents experience sleep deprivation and poor sleep quality, which may increase mental health risks of new parents and reduce the quality of caregiving behavior. Prior work in non-parent samples suggests that sleep quality may be linked to the structural connectivity of the brain, as measured within white matter microstructure (WMM). Less efficient connectivity of WMM has also been associated with mental health problems. This project will examine postpartum sleep quality in first-time fathers and test associations with their postpartum mental health and WMM. The project also examines postpartum WMM as a mediator between sleep quality and mental health symptoms during the transition to parenthood. This project draws from an existing longitudinal study of first-time parents followed from the prenatal to postpartum periods that incorporated neuroimaging. Study aims are to: (1) Examine how postpartum sleep quality, controlling for prenatal sleep quality, affects psychological adjustment to parenthood in new fathers; (2) Investigate whether sleep quality is associated with postpartum white matter microstructure (WMM) among first-time fathers; (3) Examine the mediating role of postpartum WMM on the relationship between postpartum sleep quality and fathers’ adjustment to parenthood. Study findings can provide insight into the role of sleep on the paternal brain and mental health and inform interventions facilitating the adjustment to parenthood. The proposed project will support the applicant’s training in biobehavioral research methods to become an interdisciplinary health and family studies researcher.