Earworms are persistent (mostly unwanted) mental repetitions of musical phrases. The phenomenon is also known as Stuck Song Syndrome and INvoluntary Musical Imagery (INMI). Surveys have shown the phenomenon to be very widespread - more than 90% of the general population say they have experienced them.

A number of research projects have investigated the syndrome, but as yet the cause is unknown - as is the 'cure'. Although one 2015 study at the University of Reading, UK, suggested that chewing gum might help.

Another study, also conducted in 2015 used 'Big Data' files derived from Twitter to investigate the prevalence of INMI :

We uncovered evidence that the earworm experience is a widespread psychological phenomenon reported in locations throughout the globe. We found that users openly discuss the types of music that they experience as earworms and potential causes and cures for these via their Twitter network. Finally, we discovered that people discuss INMI in more negative emotional terms on Twitter than other topics, including music in general.”

Source : ‘Catching Earworms on Twitter: Using Big Data to Study Involuntary Musical Imagery’ Music Perception, Volume 33, Issue 2.

Further reading at Wikipedia