I can empathize with your situation. "too long to read" post coming up!
I live in an apartment too and I had my fair share of noisy neighbors. The neighbor above my flat would make creepy noises at 2, 3 A.M in the morning. These noises are pretty common during the day, and one would pay no attention to it, but when heard especially after the witching hour, the eerie silence would naturally amplify them to a frightful din.
There weren't any stomping or bumping. It fact it was marbles. My neighbor would pick a marble from the bag it is kept in, and slowly drop it on the floor. The marble would then hit the ceramic flooring, creating a distinct tick sound. The force would propel the marble up and the whole sequence repeats itself thanks to gravity. Tick, tick, tick, tick. The marble would eventually lose its bouncing effect after a while and to my horror, would seem to roll off infinitely, never stopping.
This frightened the shit out of me. My brother, who is a year older, was just as equally disturbed by the queer noises coming above us. For a few days, we speculated that our ceiling could have been haunted by a dead ten-year-old.
My brother suffered with me in the sleeping department. We would lose our sleep thanks to nightmares the noises created. Our studies were affected, our mood changed. We even started taking sleeping pills, hoping to tranquilizer ourselves into deep sleep when the nightmarish ticking began.
After several weeks of tolerating, my brother grew a pair while I kept mine hidden and decided to confront the neighbor about this. I accompanied him because I secretly wished the monster, witch or whatever living above us would devour him and I loved to be there (we had a falling out that day).
He rang the door bell, and we waited, in trepidation. The door opened, and right in front of us, was a disheveled woman, in her mid 20s. I would love to describe her here but I'm going to cut this short. The tired woman inquired about our presence and my brother, obviously less afraid now, demanded an explanation for the "din". She sighed first and immediately, she apologized profusely.
She showed us into her flat and brought us into a room where a boy (aged 8 if I had to make a guess) was sitting on a queen-sized bed, staring randomly into space. "He's ill in the head," she said. As a single mum and a nurse (?), she would work a 12 hour shift every day. Her mentally slow son would be under his grandma's care in the day. She would pick him up, have dinner and put him to sleep straight after that. For some reason, he would get up and sneak out of his room. After removing his one and only marble from his toychest, he would find himself to a spot (usually above my bedroom) and proceed to entertain himself with the marble.
My brother and I felt weird after that. It was kinda awkward for some reason. We never really talk about it after that. How did I deal with it?
I just made some minor changes to my life because I know the child could never change his.
1.56 A.M. over here. I just heard my first tick for the day.