It's a normal part of the falling asleep pattern. Many people (normals, bipolars, etc) may not remember unless they are studying their dreams so self-programmed to. But we fall asleep in stages, asleep briefly then awake several times before falling into the early steady sleep. Fragment of mental audio/visual may drift thru--sometimes even startling us awake.

If you're interested in following this up, post it in the sleep/dreams forum. They know tons!