What I'm about to type is purely hypothetical and I am simply writing this to get some ideas flowing because I don't know where to start on this one. This is a peculiar case....

ok, so the stomata of the leaves are open before the light turns on and the leaves are perky. This makes me think that they are perking up because they are gaining more rigidity from gas absorption.

The light turns on, and the leaves droop again. This makes me think that the heat from the light is sapping the water from the leaves and causing them to droop. The reason they don't withhold their rigidity when the lights go on is because it isn't absorbing enough water...

Anybody have any diagnostic suggestions? This may scream root rot but I have no idea...I would think we'd see other external symptoms on the foliage by now...