(UPDATE) See my latest post for election results.
(UPDATE) It turns out that Tottori Prefecture in Chugoku Region closed the polling stations as early as 4PM (Yomiuri Shinbun). The reasons? To alleviate the undue burden on the witnesses at the polling stations (i.e. save money) and start counting sooner (i.e. save money).
ALL polling stations in Fukushima Prefecture close at 6PM, according to Sankei Shinbun. 85% of polling stations in Akita, 64% in Iwate, 57% in Miyagi will also close early. The reasons? To save energy (yes, we have to fight global warming) and to be safe, as winter is cold and dark.
According to Yomiuri Shinbun's December 10, 2012 article, more than 99% of 953 polling stations in Gunma Prefecture will close early, instead of being open until 8PM like everywhere else in Japan.
Yomiuri says the municipalities in Gunma have always been like that, closing the stations earlier so that they could start counting the votes right at 8PM and announce the result first in the nation.
From the article, it is only one municipality (Minakami) that will keep the polling stations open until 8PM. 17 municipalities will close at 5PM, 11 will close at 6PM, 23 will close at 7PM.
There seem to be two reasons:
Why waste money keeping the stations open? We can close early and start counting early, so everyone can go home early.
The sun sets early and it gets dark early in the winter, so it's dangerous on the streets.
Uh... it is as if they were still in pre-Meiji era with no electricity...