Cpw, having averaged the leaf temp and surrounds, room temp and have my offset and have a chart to look at.

This all makes sense, but what happens at night? The humidity rockets and it drops 10 degrees(the light) now the vpd is way off. How does it all work after dark? I understand the dynamics that at night vpd isn't as important because respiratory changes, but how can you control the change so it isn't so dramatic on the plants. I think this is the droop you see before lights out, changes to respiration in anticipation of sunset. 

As always it isn't critical just want to learn more about it. Never cared about this before and the girls did fine. But I think I can do better and this might be the ingredient that tames the environment and makes it friendlier to the plants. They say better controlled environment gives better results. Tell that to mother nature!😄