Romeo et al. report [1]:
"The rats exposed to bright light for 20 days or 90 days showed a relatively greater number of neuromelanin-positive neurons. Surprisingly, TH-positive neurons decreased progressively in the substantia nigra reaching a significant 29% reduction after 90 days of continuous bright light exposure. This decrease was paralleled by a diminution of dopamine and its metabolite in the striatum. Remarkably, in preliminary analysis that accounted for population density, the age and race adjusted Parkinson's disease prevalence significantly correlated with average satellite-observed sky light pollution."
Reference

The epidemiology is a spatial correlation between light pollution and Parkinson's prevalence in the US, the Willis paper: a technique seen here before.

Clearly, if there is a causal relationship between light and PD, there are implications for sleep and the use of computers etc..

Perhaps, and this is just my speculation, this helps to explain the increase of PD over time as we follow natural light patterns less and less.

It doesn't explain the high prevalence of PD in the Amish communitity (see Rick's thread).

Reference

[1] "Bright light exposure reduces TH-positive dopamine neurons: implications of light pollution in Parkinson's disease epidemiology"
Stefania Romeo,Cristina Viaggi,Daniela Di Camillo,Allison W. Willis,Luca Lozzi,Cristina Rocchi,Marta Capannolo,Gabriella Aloisi,Francesca Vaglini,Rita Maccarone,Matteo Caleo,Cristina Missale,Brad A. Racette,Giovanni U. Corsini,Roberto Maggio
Nature, March 2013
http://www.nature.com/srep/2013/1303...srep01395.html

John