Serotonin "regulates executive function, sensory gating, and social behavior" (https://pubmed.ncbi.nlm.nih.gov/25713056)

"Brain serotonin is synthesized from tryptophan by tryptophan hydroxylase 2, which is transcriptionally activated by vitamin D hormone. Inadequate levels of vitamin D (∼70% of the population) and omega-3 fatty acids are common, suggesting that brain serotonin synthesis is not optimal"

"vitamin D maintains extracellular fluid serotonin concentrations in the brain, thereby offering an explanation for how vitamin D could influence the trajectory and development of neuropsychiatric disorders" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6042449/)