@yerrag Sorry to hear about your troubles, I wish you well and a quick recovery!
Never trust GenAI about nuanced, subtle, or fuzzy topics. GenAI doesn't think or reason; it just makes statistical inferences between words.
In technical terms, fennel seeds can increase serotonin, but practically not - the amounts are insignificant!
(It is about weak MAO inhibitions)
https://pmc.ncbi.nlm.nih.gov/articles/PMC3521852/
The most probable mechanism is significantly increasing GABA, which should help with deep sleep.
https://ppj.phypha.ir/article-1-664-en.html
BTW be careful with fennel it can also increase estrogen...
Substances known to potentially lower serotonin: S. Boulardii, lavender, Shilajit, beta-alanine, Ca, Mg, glycine, lysine, Tyrosine/DLPA + BCAA, Aspirin, vitamin B9, B1, quinones, caffeine, creatine, Schisandra, inosine, theanine, propolis, Siberian ginseng, PRL-8-53, LSD, and Manganese; avoiding PUFAs…