Maternal relationship instability, particularly the dissolution of a relationship, is linked to changes in the home (e.g., parenting, inter-parental conflict) that can lead to decreases in the emotional security of the child. The resulting emotional insecurity may increase emotional and physiological arousal, which may serve to reduce the child's sleep quality and, subsequently, his/her wellbeing. This study tested a number of hypotheses informed by prior research concerning the dissolution of parental relationships. The aims of the current study were to: 1) Develop a childhood poor sleep index from the Fragile Families and Child Wellbeing (Reichman, Teitler, Garfinkel, & McLanahan, 2001) dataset informed by current research on child sleep quality; 2) Using the newly developed childhood poor sleep index, evaluate a parental relationship dissolution's effect on the childhood poor sleep index; and 3) Evaluate the child's score on the childhood poor sleep index as a mediator of a parental relationship dissolution's effect on child development. Aligned with the proposed hypotheses, in cross-sectional analyses, a maternal relationship dissolution was related to poorer child sleep quality, and this effect did not differ by type of dissolution (e.g., cohabitation versus dating relationship; social father versus biological father leaving). There was evidence of indirect effects from maternal relationship dissolutions to child development through poor sleep quality when the child was 5 years old; however, these indirect effects did not generalize to our longitudinal models of mediation. In addition, there was not a pattern of effects by child sex, however, consistently, relationship dissolutions and sleep quality had stronger negative impacts on higher SES and ethnic majority children's development in comparison to lower SES and ethnic minorities, respectively. The current findings point to sleep hygiene as a promising area of intervention among children whose parents are separating or divorcing.