You have described exactly what I’ve been dealing with for six months now. I feel it in my chest area too, along my collar bone and mostly on my left side. I was sure it was my heart but a holter didn’t show anything. It only happens when I’m falling asleep or when I slightly wake up during the night or when I’m waking up in the morning. As soon as I’m fully awake, it goes away and I don’t ever have it during the day. In the morning, especially, when I’m half sleeping and half waking up and stretching, it seems to trigger it. It happens no matter what position I’m laying in. I’m so relieved to read your experience because I have been worried about what this is for so long but like you, it doesn’t seem to be turning into anything more serious and I’ve ruled out cardiac issues. I should add that I am 42 in generally healthy. I don’t smoke, I don’t drink, I don’t consume caffeine and I try to walk daily. I’m not overweight.