Pattern of drooping

Lights on- plants are happy, healthy, fine.

By the end of the day they look like they have given up, limp from the top down.

I have yet to find the exact time threshold, but I do know it is sometime after hour 12 of 18.

And even at hour 12 they are in great condition. After 6 hours of dark, they look great again.

Light stress? Not sure what to make of the whole process.

weird, if they are limp like falling over then throw in the towel and start again.
Pictures would help.

how hot is your soil getting try a soil thermometer. always try to water at the temp of ur soil

sounds normal, leaves will go down as they are preparing for the darkness tot come and don't need their leaves to be at max absorbtion

My plants do this also.. I have 2 tents with completely different lighting, etc, and by the end of the day, everyone has a bit of a droop to them, when they had been UP and Happy all day. All I could find is the usual "They are gearing up for their dark period",, So that's what I am going with. Oh and I grow in Hempy Buckets.