Why are my plants wilting?

The past few days my plants have been going through periods where all the leaves are wilting pretty badly (typically during the dark cycle but sometimes during light too). Then they'll pop back up just as quickly and look really healthy, no yellowing at all but some redness on the branches. It doesn't seem to have to do with the watering schedule. I can't figure it out so maybe you guys can help me, here's some info about the grow:
-Plants are about 12" tall
-SOG in a flood table using hydroton
-flooding 3X during the light cycle and once in the middle of the dark (the first time they wilted I think it was because they got too dry during the dark period)
-temps in the mid to low 70s and humidity yesterday was 60 - 75%.
-600W light about 8" above the canopy
-Nutes are at about 600ppm (ECX500)
Could it be under fertilization?

I have had plants that droop a little bit during the dark cycle and then perk right back up when the light is on them. Are the drooping or actually wilting (eg stems and leaves becoming softer than they should)? Drooping can also be a sign of persistent over watering.

Yeah they're drooping more than normal during the dark cycle. Sometimes they perk back up after a watering during the dark cycle, but not always. Should I have to water more than once during the dark period in hydroton?