Overlearning of non-native speech sounds does not result in superior consolidation after a period of sleep

Recent studies suggest that sleep-mediated consolidation processes help adults learn non-native speech sounds. However, overnight improvement was not seen when participants learned in the morning, perhaps resulting from native-language interference. The current study trained participants to perceive the Hindi dental/retroflex contrast in the morning and tested whether increased training can lead to overnight improvement. Results showed overnight effects regardless of training amount. In contrast to previous studies, participants in this study heard sounds in limited contexts (i.e., one talker and one vowel context), corroborating other findings, suggesting that overnight improvement is seen in non-native phonetic learning when variability is limited.