Jenny, UK

As somebody who has very similar problems, but found a partial solution, I thought I would briefly tell you just in case it is helpful

Like you I gave up smoking after quite a number of years and like you I
don't cough when I'm asleep: conclusion : there is nothing wrong with my
lungs: the whole thing is to do with my throat, (which I have possibly
sensitised/damaged as a result of the smoking which is an added trigger).
But why?

When my doctor didn't come up with anything, being an academic I sat down and thought about it logically.
What is different between asleep and awake?
My eyes are open/closed.
I then experimented, and low and behold, if I close my eyes for say 15
minutes during the day, away from strong light, I don't cough either.
The whole thing is to do with the amount of 'tearing' of my eyes.
So that is what needs dealing with.

The second thing is that I discovered that some people do what they call "cry through their nose".
It is a psychosomatic phenomenon due to suppressed sadness. Some people have a constantly blocked nose, others, like me have the constant teary trickle which causes the cough. One person I met had to use nose-drops several times a day until her father who had abused her died, and from that day on never had to use the drops again.

So I declined all lung tests or treatments and concentrated on my eyes instead, sunglasses in the morning, and I make sure I never get cold, even indoors. etc. The only time I now cough significantly is when I yawn a few times and my eyes fill with tears.

I blow my nose a lot, as soon as there is any hint that something could be trickling down. That helped a lot as well.

Have a think and see what happens !! Good luck!