Sleep problems are increasing in modern society. Throughout history, lullabies have been used to soothe the sleep of children, and today, with the increasing accessibility of recorded music, many people report listening to music as a tool to improve sleep. Nevertheless, we know very little about this common human habit. In this study, we elucidate the characteristics of music used for sleep by extracting the features of a large number of tracks (N = 225,927) from 989 sleep playlists retrieved

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... om the global streaming platform Spotify. We found that compared to music in general, music used for sleep is softer and slower; it is more often instrumental (i.e. without lyrics) and played on acoustic instruments. Yet, a large amount of variation was found to be present in sleep music, which clustered into six distinct subgroups. Strikingly, three of these subgroups included popular mainstream tracks that are faster, louder, and more energetic than average sleep music. The findings reveal previously unknown aspects of sleep music and highlight the individual variation in the choice of music for facilitating sleep. By using digital traces, we were able to determine the universal and subgroup characteristics of sleep music in a unique, global dataset. This study can inform the clinical use of music and advance our understanding of how music is used to regulate human behaviour in everyday life.