I don't know if I'll ever figure out my sleep disturbances, but one reason seems to be increased norepinephrine (NE).
The easiest cause is if I have too much caffeine. Sometimes it will delay sleep, but not always. I'll wake up at 2:00 - 3:00 a.m., with a struggle to fall back if at all.
This also happens with infection/inflammation, which can raise NE. An odd aspect of NE is that some microbes use it for growth.
When I'm treating an infection, or, if I'm chelating metals, I will have disturbed sleep from what I would describe as a release of endotoxin.
Increase in inflammatory cytokines may also be involved :
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3548567/