Autism

You're welcome. Unfortunately there's no one place where you can read up on it; in my case I searched for any studies or papers on autism and/or serotonin to get a better understanding of how the system worked. Whilst some research is behind paywalls, a lot is still publicly accessible (ResearchGate is one such site). Google Scholar is probably the easiest way to search for relevant papers, though, as it usually lists where the full paper (as a PDF) is available.

Some of the papers I've come across that may be of interest to you:

• Serotonin is a key factor in motor function - Jacobs, B.L. and Fornal, C.A. (1999) ‘Activity of serotonergic neurons in behaving animals’, Neuropsychopharmacology: official publication of the American College of Neuropsychopharmacology, 21(2 Suppl), p. 9S–15S.
• Serotonin seems to play a role in "hot" vs "cold" processing; this may be why emotional processing is affected in autism - Robinson, O.J. and Sahakian, B.J. (2009) ‘A double dissociation in the roles of serotonin and mood in healthy subjects’, Biological psychiatry, 65(1), pp. 89–92.
• One form of the serotonin transporter is linked to increased sensory sensitivity, often found in autism - Homberg, J.R. et al. (2016) ‘Sensory processing sensitivity and serotonin gene variance: Insights into mechanisms shaping environmental sensitivity’, Neuroscience and biobehavioral reviews, 71, pp. 472–483.
• Vitamin D's role in serotonin synthesis and relevance to autism (this paper notes that CNS serotonin is lower in autism than in NTs) - Patrick, R.P. and Ames, B.N. (2014) ‘Vitamin D hormone regulates serotonin synthesis. Part 1: relevance for autism’, FASEB journal: official publication of the Federation of American Societies for Experimental Biology, 28(6), pp. 2398–2413
• Serotonin-NAS-Melatonin pathway affected in autism (Melke et al in particular notes the genetic basis for this) - Pagan, C. et al. (2014) ‘The serotonin-N-acetylserotonin-melatonin pathway as a biomarker for autism spectrum disorders’, Translational psychiatry, 4, p. e479., Martinez-Cayuelas, E. et al. (2022) ‘Melatonin Rhythm and Its Relation to Sleep and Circadian Parameters in Children and Adolescents With Autism Spectrum Disorder’, Frontiers in neurology, 13, p. 813692. and Melke, J. et al. (2008) ‘Abnormal melatonin synthesis in autism spectrum disorders’, Molecular psychiatry, 13(1), pp. 90–98.
• The TPH2 gene (which controls the rate-limiting enzyme for serotonin synthesis in the brain) may be involved in autism - Coon, H. et al. (2005) ‘Possible association between autism and variants in the brain-expressed tryptophan hydroxylase gene (TPH2)’, American journal of medical genetics. Part B, Neuropsychiatric genetics: the official publication of the International Society of Psychiatric Genetics, 135B(1), pp. 42–46.

As for the quantity of melatonin, that was in discussion with my doctor. He expected that the commonly available 2mg supplements wouldn't do anything and that it would probably take 10mg to have a significant impact. He also said not to go above 20mg. In my case, 10mg works very well, but was increasing the risk of being tired the next day - so I am currently on 5mg (this helps, but I don't sleep through the night as well as I did on 10mg).

Also on the safety side of things, be aware that 5-HTP should not be taken with SSRIs without careful medical supervision due to the interaction and risk of serotonin syndrome.