I'm just going to make some shit up to see if it sticks so let me know what you think. When the light is on your plants are in a production cycle, processing food producing chlorophyll and hydration so the plant is in an active cycle. When you water during this time the plant draws moisture into the leaves more efficiently weighing down the leaves causing them to droop. When you water after dark the uptake of moisture is slower so when the lights come on the plant has already processed the excess water. Sounds good to me but I really don't know. Lol