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 = .062) and receptive vocabulary growth (p = .006), whereas the length of night‐time sleep was negatively associated with rate of predicted expressive vocabulary growth (p = .045). Sleep efficiency was also positively associated with both predicted receptive (p = .001) and expressive vocabulary growth (p = .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.