I have a friend who regards this experience as being "visited by the song fairy". I so loved this explanation that I wrote a song about it.

THE SONG FAIRY

LAST NIGHT THE SONG FAIRY CAME TO CALL
SHE CAME WHEN I WAS FAST ASLEEP
SHE BARELY MADE A SOUND AT ALL
AS INTO MY MIND SHE DID CREEP
SHE FILLED MY HEAD WITH SONGS SO FINE
THAT IN THE MORN I WOKE UP SINGING
SHE SAID THAT I COULD MAKE THEM MINE
AND WHEN SHE LEFT MY HEAD WAS RINGING

THE WORDS, THOUGH MIXED AND SPINNING ROUND
CAME GUSHING FORTH AT RECORD PACE
AND AS I QUICKLY WROTE THEM DOWN
WELL, OTHERS CAME TO TAKE THEIR PLACE
THEY TOLD OF SORROW AND OF LOVE
OF CHILDHOOD MEMORIES AND OLD AGE
OF NATURE AND THE HEAVENS ABOVE
AND SO THEY FILLED PAGE AFTER PAGE

THE TUNES, THEY CAME JUST LIKE THE WORDS
NOTE AFTER NOTE, EACH FOLLOWED ON
'TIL IN MY HEAD A VOICE I HEARD
AND IT WAS SINGING EVERY SONG
LAST NIGHT THE SONG FAIRY CAME TO CALL
SHE CAME WHEN I WAS FAST ASLEEP
SHE BARELY MADE A SOUND AT ALL
AS INTO MY MIND SHE DID CREEP