Hey bud!! Without baffling with science, have a look at DLI, if your hitting them hard with the light and they start dropping towards the end of the light cycle...... With no reason, they are happy, fed, watered etc and they droop then DLI could be a factor, basically if it is DLI the plant needs to 'sleep' as its had enough for one day, but make sure it is and not overwatering etc first, food for thought!!
Hope this helps, happy growing 👍