Just to add my experience, there seem to be two types of droop that happen near the end of the light cycle. There is a droop that happens to all my plants that looks like they are standing at attention with arms to side when lights out (happens to them all, it seems like sleep?). Also, I have noticed that there is a droop where both the stem and leave droop together and doesnt look as rigid as their regular "sleep droop". This second type is usually an indicator to me that light saturation has been reached for a plant. You will also notice the temperature rise if the droop is caused by light saturation. Just what I have seen in my grows.
