Adults learning a new language are faced with a significant challenge: non-native speech sounds that are perceptually similar to sounds in one's native language can be very difficult to acquire. Sleep and native language interference, 2 factors that may help to explain this difficulty in acquisition, are addressed in 3 studies. Results of Experiment 1 showed that participants trained on a non-native contrast at night improved in discrimination 24 hr after training, while those trained in the morning showed no such improvement. Experiments 2 and 3 addressed the possibility that incidental exposure to perceptually similar native language speech sounds during the day interfered with maintenance in the morning group. Taken together, results show that the ultimate success of non-native speech sound learning depends not only on the similarity of learned sounds to the native language repertoire, but also to interference from native language sounds before sleep.