Plants have a period of rest. They will generally droop and look somewhat drought stricken almost about 3 hours before lights out. They will remain that way till lights out and several hours into the dark period. About 2 hours before lights on they perk up and are standing totally erect and reaching for the light.
Why it occurs this way, i don't know, but that's what my plants do. I have night vision cameras in my grows so i can see what they do 24/7