Related to this: Since when I was young, maybe around six or so, on nights before special days I looked forward to a lot (like my birthday), I was desperate and agonized over the realization that my current self (what you call "train of thought") would end once I fell asleep, so this current longing that I had would never actually be fulfilled. This made me really sad, and it felt like my current "self" would die and get replaced the next morning by a somehow related "self", that was in it's essential emotional state very new and in that regard definitely separate. I continued to have this feeling from time to time (maybe 2-3 times per year, less later on) until well into my twenties, when it somehow receded. I have not talked about this often, but when I did, I never found anybody that could relate.