Plants will also droop (relax) when they have had enough to eat for the day. I read up on daily light intregal and ran some tests.
At just over 220 watts/sqaure foot the plant would relax after 15.5 hours of light, dropping the light to 200 watts a square foot increased the time to 16 before droop. Under 150 watts/foot the plant would eat all day. This was a bagseed sativa I still grow.
I personally like the plant responses with the higher intensity light and short cycles, but actual bud yield does not seem to care.
Like a resting human having low blood pressure, an inactive plant has low water pressure, causing the leaves to not stand as straight.