Background: The facilitating role of sleep for language learning is well-attested in adults and to a lesser extent in infants and toddlers. However, the longitudinal relationship between sleep patterns and early vocabulary development is not well understood. Methods: This study investigates how measures of sleep are related to the development of vocabulary size in infants and toddlers. Day and night-time sleeping patterns of infants and toddlers were compared with their concurrent and subsequent vocabulary development. Sleep assessments were conducted using a sleep diary specifically designed to facilitate accurate parental report. Sleep measures were used as predictors in a multilevel growth curve analysis of vocabulary development. Results: The number of daytime naps was positively associated with both predicted expressive (p = 0.062) and receptive vocabulary growth (p = 0.006), whereas the length of night-time sleep was negatively associated with rate of predicted expressive vocabulary growth (p = 0.045). Sleep efficiency was also positively associated with both predicted receptive (p = 0.001) and expressive vocabulary growth (p = 0.068). Conclusions: These results point to a longitudinal relationship between sleep and language development, with a particular emphasis on the importance of napping at this age.