Normal for plants to droop just before their dark cycle??

Everything has been going great so far, however, I've noticed usually 2-3 hours before the light goes out, my plants will start to droop (mainly the Afgooey). Is this normal? When should I start giving them nutes? They are probably abt 3 weeks old. Any input would be appreciated.

1500W HPS
18 on/6 off
Watered every other day in 1 & 2 gallon pots.

Normal, normal.

from what i've seen on youtube videos....yes. its actually pretty neat. i'm nowhere near an expert or botanist, but it appears almost as if branches and leaves oscillate up and down, almost like its breathing in and out. yeah, right before dark they crinkle and aim down. when light appears - or when it prepares for the light - it reaches back up

Cool deal. Thanks you 2.