When toddlers hear a novel word, they quickly and independently link it with a novel object rather than known-name objects. However, they are less proficient in retaining multiple novel words. Sleep and even short naps can enhance declarative memory in adults and children and this study investigates the effect of napping on children's memory for novel words. Forty two-and-a-half-year-old children were presented with referent selection trials for four novel nouns. Children's retention of the words was tested immediately after referent selection, four hours later in the afternoon, and the following morning. Half of the toddlers napped prior to the afternoon retention test. Amongst the toddlers who napped, retention scores remained steady four hours after exposure and the following morning. In contrast, for the wake group, there was a steady decline in retention scores by the following morning and significantly lower retention scores compared to the nap group. Napping following exposure to novel word-object associations could help in maintaining memories and limiting decay. Nap duration was also associated with better retention scores, but there were no effects of sleep quality, habitual napping, or sleepiness. The findings have implications for the role of napping in children's language acquisition.

Children can easily link a novel word to a novel, unnamed object—something referred to as fast mapping. Despite the ease and speed with which children do this, their memories for novel fast-mapped words can be poor unless they receive memory supports such as further exposure to the words or sleep. Axelsson, Swinton, Winiger, and Horst (2018) found that 2.5-year-old children who napped after fast mapping had better retention of novel words than children who did not nap. Retention declined for those who did not nap. The children received no memory supports and determined the word–object mappings independently. Previous studies report enhanced memories after sleeping in children and adults, but the napping children’s retention in the Axelsson et al. study remained steady across time. We report a follow-up investigation where memory supports are provided after fast mapping to test whether memories would be enhanced following napping. Children’s retention of novel words improved and remained greater than chance; however, there was no nap effect with no significant difference between the children who napped and those who did not. These findings suggest that when memory supports are provided, retention improves, and the word–object mappings remain stable over time. When memory traces are weak and labile, such as after fast mapping, without further memory supports, sleeping soon after helps stabilise and prevent decay of word–object mappings.