The following quote is from Melev's Reef, (http://melevsreef.com/rk101_sumps.html)which has excellent articles to answer any question you could think of re: sumps and refugiums.

"The refugium will need some type of light over it for the macro algae to grow. My preference is a 5100K spiral compact fluorescent bulb. I run the lighting on a reverse photo period, from 10pm to 8am daily. (When I used to keep Caulerpa nummularia, I found that leaving the light on 24 hours a day prevented it from going sexual. It was an easy solution to an annoying problem.)"