An aneurhythm is an involuntary imagination in which one seems to hear a repeating melody or musical phrase, like it's "stuck in the head." I call them "aneurhythms" rather than "earworms" because they've kept me up for hours; it would be wrong to call them anything but symptoms. If I have one when I'm awake, it helps to play the song and then follow it with something equally catchy. I've never been able to figure out why that works. It's like the brain would rather go silent than think two conflicting thoughts at once.
Usually, though, aneurythms are much worse at night. The first time I ever saw the sunrise was in 3rd grade, after we sang "You're A Grand Old Flag" in class and I stayed awake, with it repeating in my mind's ear, until breakfast. I didn't know what to call it then, so out of superstition I began to avoid patriotic tunes. Years later, when I got a new phone with "God Bless America" set as the default ringtone, I had flashbacks. Meanwhile, other catchy choruses had become fixtures of my insomnia and my nightmares.