I like the serotonin as police theory. Serotonin is involved in so many things. I think that what it really does is regulates other neurotransmitters, which in turn regulate other substances, processes, feelings, and indirectly actions. But the whole process can operate from the reverse too. Actions can affect feelings and change the substances and proportions and activity in the brain. Exposure to environmental conditions is important too.

It seems to be established that light exposure regulates serotonin, and serotonin regulates melatonin and the sleep/wake cycle. Light and dark can be used effectively to treat depression, mania, and sleep or circadian rhythm disorders, as long as you control the light exposure in the right ways. Mood is connected somehow to serotonin, and to this cycle.

We can also establish that dopamine is connected somehow with mood and perception, maybe in a different way. Too much dopamine is associated with schizophrenia, and too little with Parkinson's disease. We also know that medications that control schizophrenia symptoms can cause Parkinson's-like symptoms. I wonder if Parkinson's meds could trigger schizophrenia too?

There is so much that we don't know. I find the connections between one thing and another to be very interesting.