What is it?

Here I am at 2 am awake  - I did go to bed two hours ago - but I'm back again sitting at my computer. My mind doesn't seem to be functioning enough to write this post without my fingers hitting keys that aren't meant to be hit at the time I'm hitting them let alone my novel. But it's calling and I'll write - perhaps incoherent drivel but get words on the page none the less.
I can fix it later. Or delete it. Whatever works best when my mind can think again.
What is it with writing that wakes you up in the middle of the night and drags you out of bed to work on a novel that may never be read by anyone but yourself (and maybe a few close friends). Is it the hope that it will in fact be published and read by thousands (I'd say millions but my hope is never set that high). Or is it your character alive inside your brain just waiting to be reborn on paper?