Can someone explain this phenomenon to me, it makes no sense to me

I make a song, in one day, I'll spend 6 hours on it, very actively.
I finish working on it (for the day being)
Until the last minute before falling asleep that same day, i will hear that song in my head non stop.
Now mystery #1: i wake up, and i don't know what the song was. I cannot recall a single note, section, instrument sound, as part of the song. Nothing. Until i physically hear it again.
Now mystery #2: after hearing it once on any day after the first day, and then sleeping after that, i wake up the following day and i CAN recall the song

This has now happened consistently like 50 times
What is this brain thing