A Song in my Heart...er...Head

I have a question...does anyone ever wake up with a song stuck in their head? As in, it wasn't there when you went to bed, but it was there when you woke up? For as long as I can remember I have. Almost every single morning there is a new song just stuck in my head. Sometimes it's a song I like & sometimes it's not. Sometimes it's a song I heard just the day before & sometimes it's one I haven't heard in years. I assumed that it was something that happened to everyone, but the other day I was talking to my friend Dale (who happens to be as passionate about music as I am) and we somehow got around to this topic of conversation and I was shocked to find out that he DIDN'T wake up with a song in his head. So I started asking some more people...so far 6 out of 6 have said "nope" no song in their heads in the morning either. Interesting. I like having a song in my head in the morning, I wonder why I do? And how did it get there?