This used to happen to me when I was a young teenager (I will be 24 this month) and has recently started happening again.
I don't know how to explain it properly, but when I wake up in the morning I have a very bad "furry teeth" feeling and very bad taste in my mouth, and my front two teeth have this brown stuff on them that can easily be brushed or wiped off. I also dribble a bit in my sleep and if I dribble onto a light coloured pillow it leaves brown patches on it.
It's really disgusting and I don't know what is causing it or how to stop it from happening. I brush morning and night and generally do try to keep good care of my teeth but this keeps happening!
I have the same problem as well. The answer is simple- even though it took me almost a year to figure out- cut out dark sodas and drink plenty of water.. This will fix the problem and will also have a few more desired health effects. Hope this helps.