Music can evoke strong emotions und musical chords are the smallest musical units that carry emotional information. When we are awake, major chords are typically associated with positive emotions, while minor chords tend to evoke negative ones. But does the human brain also differentiate between musical chords during sleep? Researchers at the University of Fribourg are addressing this largely unexplored question.

Even during sleep, the brain does not simply switch off; it continues to process external input, albeit at a reduced level. Listening to music can provoke strong emotional reactions, but whether this also occurs during sleep was previously unknown. Researchers have now taken an important first step, demonstrating that the sleeping brain is indeed capable of distinguishing between different musical chords.

Inside the Sleep Laboratory
Professor Björn Rasch and his team at the University of Fribourg investigated this question in a recently published SNF-funded study in the journal Sleep Advances. In this proof-of-concept study, the researchers examined whether the sleeping brain can differentiate between various categories of musical chords.

47 young adults participated in the study and spent a night in the University of Fribourg’s sleep laboratory. While they slept, the research team played major, minor, and dissonant chords, while an EEG recorded the participants’ electrical brain activity.

Musical chords are differently perceived during sleep
The analysis of brain responses (ERPs) revealed clear and significant differences among the three chord categories. Major chords produced the strongest amplitude roughly 800 milliseconds after the chord began. Minor chords elicited intermediate amplitudes, whereas dissonant chords produced the lowest amplitudes. Notably, individuals with experience playing a musical instrument showed stronger differentiation between the chord categories during sleep compared to participants without musical training.

The researchers conclude that the differential processing of single musical chords persists during sleep and is shaped both by harmonic properties as well as an individual’s musical expertise within the framework of Western musical conventions. Future studies should investigate whether longer and more complex harmonic structures – such as chord sequences or entire musical pieces – are similarly differentiated by the sleeping brain, and whether the presentation of emotionally positive musical elements during sleep can help to improve sleep quality.

HarmonicSleep Project
​This study is part of an SNF-funded project «HarmonicSleep» (Project number 10.002.583). The project investigates whether the sleeping brain can process music and sounds. In addition, it examines whether music and sound processing during sleep can improve sleep quality and mental recovery.

Study
Anna Zoé Wick, Björn Rasch, Musical harmony is processed during sleep: a proof-of-concept study, SLEEP Advances, Volume 6, Issue 4, 2025, zpaf085, https://doi.org/10.1093/sleepadvances/zpaf085