Nighttime sleep awakenings and infant and family characteristics were measured longitudinally in more than 1,200 infants when the infants were 6, 15, 24, and 36 months old. By 6 months of age, the majority of children slept through the night, awakening their mothers only about once or twice per week. However, not all children followed this predominant developmental pattern. Using growth mixture modeling, we identified 2 distinct developmental patterns. One group, labeled "Sleepers", included 66% of the children. These children showed a flat trajectory of sleep awakenings from 6 through 36 months, with mothers reporting their infant awakening from sleep about 1 night per week. The second group, labeled "Transitional Sleepers", included 34% of the infants. These children had 7 reported nights of awakenings per week at 6 months, dropping to 2 nights per week at 15 months and to 1 night per week by 24 months. Compared with Sleepers, Transitional Sleepers were more likely to be boys, score higher on the 6-month difficult temperament assessment, be breastfed at 6 and 15 months old, and have more depressed mothers at 6 months old. Using 2-group structural equation modeling, we examined individual differences at different points on the individual infants' sleep trajectories. For infants in both groups, reported sleep awakenings were associated with difficult temperament measured at 6 months, breastfeeding, infant illness, maternal depression, and greater maternal sensitivity. Infant-mother attachment measures were not related to these sleep awakenings. (Contains 7 tables, 1 figure and 2 footnotes.)