Musical harmony is processed during sleep: a proof-of-concept study

Musical chords represent the most basic musical elements conveying emotional information. During wakefulness, different musical chord categories elicit distinct neuronal correlates and emotions, with major chords typically inducing more positive and minor chords with negative ones. However, it remains unclear whether the brain continues to process musical chords differently when presented during sleep. To address this question, we conducted a proof-of-concept study and presented musical major, minor, and dissonant chords to 47 healthy participants during nocturnal non-rapid eye movement (NREM) sleep. Prior to sleep, participants rated the chords in valence and arousal. Sleep was recorded using polysomnography. Our analysis of event-related responses during sleep revealed significant differences between the three musical chord categories. Major chords induced the strongest negative amplitude approximately 800 ms after chord onset, indicated as peak-to-peak (PTP) amplitude from the earlier positive peak. Minor chords showed intermediate PTP amplitudes, while dissonant chords elicited the lowest PTP amplitudes. In the time-frequency domain, these differences were also apparent, including differences in slow-wave, theta, alpha and sleep spindle bands across chord categories. Notably, experience in playing a musical instrument induced stronger differentiation of musical harmony during sleep compared with participants who never played a musical instrument. In conclusion, our findings suggest that the different processing of single musical chords persists during sleep, influenced by harmonic features and musical expertise in the context of Western musical conventions. Future research should explore whether longer and more complex harmonic features (e.g., chord sequences or musical pieces) are differentially processed by the sleeping brain. Statement of significance Whether the sleeping brain can process musical harmony remains an open question. In this study, we investigated the perception of musical harmony during sleep. Our results demonstrate that the sleeping brain is capable of differentiating between musical chords, and that this sensitivity is modulated by the level of musical expertise. Given the strong link between musical harmony and emotional processing, these findings have important implications for the use of music to influence emotions during sleep and dreaming and may even contribute to novel approaches for improving sleep quality in clinical settings.