The present study examined both between- and within-person effects of maternal sleep patterns on quality of mothering at bedtime during infants' first 6 months. Participants included 142 mothers who reported on their daily fall asleep and wake times across seven consecutive days with a daily sleep diary when infants were 1, 3, and 6 months old. At each age point, maternal emotional availability during one night of infant bedtime was observed and scored by trained observers who were blind to maternal sleep patterns. Multilevel modeling revealed that mothers with irregular sleep patterns, especially later average fall asleep times and greater average variability in sleep period across three age points, showed poorer parenting quality with infants at bedtime than other mothers. In addition, both between- and within-person effects of maternal sleep on bedtime parenting quality changed with infant age. Compared to mothers' individual averages across 1, 3, and 6 months, maternal short average sleep period, increased variability in sleep period, and later fall asleep times predicted poorer bedtime parenting quality at 6 months, but not at 1 or 3 months. Results emphasize the importance of maternal sleep regulation and sleep hygiene for maternal parenting quality, especially as infants get older. (PsycInfo Database Record (c) 2020 APA, all rights reserved).