The sounds we use to drift off "display a large variation including music characterised by high energy and tempo," say researchers at a Danish university. Researchers from Aarhus University analysed over 200,000 songs on Spotify from nearly a thousand playlists associated with helping users to get to sleep.
Alongside the expected types of slower instrumental music with minimal elements and a calming nature, the researchers also found that many use familiar and comforting music to fall asleep to, even if they are more up-tempo and energetic.