Veg light cycle?

Hello everyone. My first post...glad to be a member. So much info.
I just got six really nice looking clones the other day. They were in rock wool and I transplanted them into soil. I have them under fluorescents and they look happy. Here is my question...should I be leaving the light on 24 hours or should I give them 18/6? Is more hours of light better? Or do they need that six hours to do "stuff"?
Any help would be really appreciated.

I would give them 24/0 for a week to get them set then18/6.. I found that my plants recover well in the 6 hours of no light and find new shoots coming out every morning. Also put a light fan on them to get the stem conditioned.

I ran mine for 24/0 the first week and now I'm on 20/4... I turn the humidifier off and put my space heater in there...keeps it at 70deg and 40% humidity...And I ALWAYS have my fan running

Right on. Thanks guys that's great advice. Will do.