The present study examined mothers' emotional availability (EA) during daytime free play and bedtime as a mediator of linkages between maternal nighttime sleep and infant-mother attachment. Participants included 153 mothers (85% White) with infants (53% female). When infants were 1, 3, 6, 9, and 12 months, maternal sleep was assessed using actigraphy and daily sleep diaries for 7 consecutive days. At each time point, mothers' EA was scored from one observation of daytime free play and from one evening observation of infant bedtime by trained observers who were blind to all other participant information. Average scores were created for maternal sleep and EA across the five occasions in the first year. At 12 and 18 months, infant-mother attachment security in the home was scored by blind observers using the Attachment Q-Set, averaged across the two age points, and used in analyses. Mediational analyses revealed that mothers who experienced highly variable sleep and had poor sleep quality were less emotionally available with infants at bedtime during infants' first year of life, which in turn was predictive of lower infant-mother attachment security in the second year, supporting mediation. Linkages between maternal sleep characteristics and daytime EA were less evident. Later maternal sleep timing was also directly predictive of low attachment security, after accounting for maternal EA. Findings emphasize that poor parental sleep places both parenting and infant socioemotional development at risk, and that parental sleep hygiene and sleep habits should be a salient focus of parenting intervention. (PsycInfo Database Record (c) 2022 APA, all rights reserved).