I've seen them at night, under the grow lights every leaf blade makes a clear angle respect the horizontal plane (not bigger then 45º I'd say). The leaf points are not curled up like on a nutrient deficiency. Also, I observed that this phenomenon doesn't last long, perhaps a few hours, because in the morning when I look at them again, the leaves are back on their horizontal plaes, and look healthy and strong, but not that conspicuous as they had been seen the night before. Really the levaes seem erect, like a morning's boner.
I agree with @chastmtz, I see this as vigorous health, not temperature (18-25ºC, led lights 'full spectrum' at 40 cm ) nor deficiency of any nutrient.
I'll try to get some pics next time.
Regards.