When you spend a lot of time working, traveling, living together with someone or group of individuals you start to discover all kinds of things about them. Some good, some bad. Isabella, Brandy and Melody have just discovered that Janet talks in her sleep.
This is from a dream I had where the girls from Move to the Music were passing the time before a promotional photo shoot. In typical entertainment-industry style it is a case of "hurry up and wait", but the girls are now seasoned veterans and have come prepared to keep themselves occupied. Isabella is tackling some Sudoku puzzles. Brandy is taking the time to do some hand polishing on the motorcycle engine she's rebuilding, and Melody is enjoying her latest book purchase. Janet seems to be catching up on lost sleep. When asked later she did not remember having a dream involving ponies so the girls have no context to explain the outburst.
Note. I had this dream more than a year before Friendship is Magic came out, however since its debut Janet has become a huge fan of the show.