The majority of adults sleep with a partner, and for a significant proportion of couples, sleep problems and relationship problems co-occur, yet there has been little systematic study of the association between close relationships and sleep. The association between sleep and relationships is likely to be bi-directional and reciprocal—the quality of close relationships influences sleep and sleep disturbances or sleep disorders influence close relationship quality. Therefore, the purpose of the present review is to summarize the extant research on (1) the impact of co-sleeping on bed partner’s sleep, (2) the impact of sleep disturbance or sleep disorders on relationship functioning, and (3) the impact of close relationship quality on sleep. In addition, we provide a conceptual model of biopsychosocial pathways to account for the covariation between relationship functioning and sleep. Recognizing the dyadic nature of sleep and incorporating such knowledge into both clinical practice and research in sleep medicine may elucidate key mechanisms in the etiology and maintenance of both sleep disorders and relationship problems and may ultimately inform novel treatments.