Hi Rod - as Alan also said before - I think (especially in my case) it's more a case of the phosphates causing the hair-algae to grow out of control, than the lighting period. Reducing the lighting period is most likely only a way to assist in controlling already existing hair-algae.....
I have noticed after reducing the feeding of my tank, a huge reduction in hair algae, and also quite a lot of dying off of hair algae.... (I also reduced my lighting period a bit as well)...
My T5's switch on at 6:00am until 10:00pm..... My Metal Halides are on: during the week, from 12:00pm to 9:00pm, and weekends, from 10:00am to 9:00pm (at this stage.... I might reduce this still a bit even - BUT I have corals and anemone's to care for still as well)....
BTW: Interesting thing though - my nitrate levels have ALWAYS stayed between 0 and 2 on my Salifert test kit..... even with the hair-algae die-off occurring...
BUT, I also dosed some more bacteria in my tank (and am trying out the vodka method - which could also possibly be working - no proof as yet)...