Important Q : Is there any research reference to describe the biosynthesis pathway as shown highlight above? What is going on how sunlight affect on body clock circuitry through eye receptors?
There's more research supporting that than there is supporting serotonin and sunlight, actually.
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Here's some excellent information pertaining to what he mentioned:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5027321/What we think about also matters to the brain, rumination is the habit of dwelling on our negative thoughts repeatedly at length. It ramps up our brain's stress circuitry, and that, in turn, interferes with the whole process of memory consolidation. It is the big reason why memory tends to suffer whenever we are stressed and we get trapped inside our own heads. The research shows that we are most likely to ruminate whenever we are completely alone
What's mentioned here tells us that our perception plays a psychological role in depression. For instance, if you wake up to a dark cloudy day, telling yourself that you can't go out today or do anything exciting because of the weather will eventually lead to that outcome (self-fulfilling prophecy). But if you see the cloudy day, and perceive the opposite, I'll make you happier because you're not allowing the temperature to dictate your mood.