re: "Say most PD patients present with sleep dysfunctionalities."

I understand that idea, but the problem with it is that I'm not just saying that PD is correlated with coexisting sleep deprivation; I'm saying that evidence suggests PD is associated with a history of pre-existing sleep deprivation, where the sleep deprivation is itself correlated with the demands of a person's job/lifestyle. It seems obvious to me that the most plausible cause-and-effect relationship there is that the sleep deprivation is causing the PD, rather than vice versa or some third factor causing both.

re: "If you investigated PD a bit further, you'd realize that there's dozens of confirmed comorbidities. That doesn't tell us anything about anything."

I've actually read the literature reviews that neuroscientists use to learn about this stuff. I even include links to those journal articles. Here's the most recent one I found: "2011.04 - Epidemiology and etiology of Parkinson’s disease: a review of the evidence." I mention this journal article in the essay on PD and sleep.

You're right that comorbidities don't necessarily tell us much. But the evidence I'm talking about seems different, because the factor I'm discussing (sleep dep / job demands) predates the PD symptoms. You could argue that an underlying cause of PD was latent in these people earlier in life (their 20s-40s) and causing them to work longer hours and get less sleep, but it just seems much simpler and more sensible to me to say that people were working harder, getting less sleep, and paying a price as a result.

"it's exceedingly more likely that DA function plays the role of causal link here than lack of sleep."

I think you misunderstood the point I was making in that section; I was writing that for people who did not know much about how the brain works. I agree it seems clear it's the DA neuron death that's causing the PD symptoms; my hypothesis is that lack of sleep over a lifetime is causing gradual DA neuron death.

re: "Fox's lack of family history doesn't suggest anything at all."

I'm not trying to establish that it's lock-tight that there was no genetic component; I'm trying to explain to the average person out there that Fox's situation was not a case where everyone in his family had PD and so he got PD too. For a long time I assumed that's what had happened. Again, it isn't a lock-tight piece of evidence, but I do think it's relevant and adds weight to the environment-side of the scale (I'm imagining a scale where genetics is on one side and environment is on the other). It just seems more likely to have been caused by his environment if his family doesn't have a history of PD. That seems totally reasonable to me, and I think it would to most people.

re: "You're connecting three semi-confirmed factoids (increased PD risk among amphetamine users, amphetamines lessen the need for sleep and are taken by traditionally sleep-deprived groups) by positing a direct causal connection; namely, amphetamines -> no sleep -> PD. All this while ignoring much more sensible connections (e.g., via dopaminergic systems)."

Again, I think you misunderstood what I was trying to get across in my essay: I was saying that the amphetamines AND / OR sleep were causing damage to the DA neurons. So it seems plausible to me that amphetamines are causing the DA damage on their own and sleep dep. has no effect, or that amph. have no effect and only cause DA damage by causing sleep dep, or that both amph. and sleep dep. are able to independently cause DA neuron damage.

Thank you again for your feedback! There are some complicated facts here to process (complicated for those who haven't studied this subject) and so it's helpful for me to see when I'm not communicating my ideas as clearly as I would like.