I too have had this for decades… many many years ago it was said to be a post nasal drip. I have recently been down for near 3 weeks with a fluey bug/ chest infection and the mucus produced ( as you say) got worse with all the coughing , I am recovering but the mucus is still here overnight if I’m not sleeping, and ALWAYS first thing in the morning. Sometimes it is so bad I retch. Once retched it is usually better, but I hate it…it’s like a permanent morning sickness, which at 80 I could do without. I have tried every so called cure under the sun. I sleep on 5/6 pillows but, as someone said, I always end up sliding down in sleep. Someone told me to put a pillow under my knees would help stop me sliding… it didn’t, but gave me a pulled muscle in my upper thigh! I do wish someone would find a fix for this. I’m not sure that heat or cold have a lot to do with it …sometimes I can go for a couple of weeks virtually free of it, then it comes back. It doesn’t SEEM to be dependant on the weather. Having said that I live in WET WALES! So even in the summer we get wet! I do wonder if it is related to Sledp Apnoea, which I was diagnosed with about 10 years ago, but I get claustrophobic with the machine, but it was better when I used it. However in 21 I was hospitalised with unrelated issues and they told me in hospital I didn’t need it, so when I came out I felt to ill to return …and never have …or have the SA symptoms … but ? Who knows?