It sounds like the brain is running something like a defragmentation routine every night. If it doesn't complete, your thoughts become more and more incoherent.
Once I had some strong personal issues and spent a full 6-7 days without getting any sleep. At that point I was convinced that my neighbors (Pakistani) were from IS, I even started an investigation (using some tactics you can guess) in order to find evidence and was already seeking political asylum in the US. Now I think about it and it's even funny, but hell.
I stayed awake for two weeks once. After the first 8ish days you start hearing and seeing odd things. Really helps with music production. Ventured into soundscapes I'll never come back to.
The longest I've been awake is 40h I think. I started hearing and seeing things at the 39th hour and that's when I decided to go to sleep.
I've got no clue how people stay awake for more than 48h; just last week I fell asleep in front of the computer after being awake for 24h. For me, it requires mental gymnastics to be able to go beyond that.
Past about 24 hours for me I start seeing things out of the corner of my eyes. Somewhat before that (on night's I get maybe 3 hours sleep) by the end of the day I get easily startled.
