During the day, your plants grow TO the sun, photosynthesising... producing a healthy looking leaf.. at night, when there's no light, no more photosynthesis, leaves have nothing to do. Stomata which are the cells on the bottom of the leave close at night forcing the water towards the stem and roots thus the droop. During the light, these cells open back up and fill with water which helps cool (asperate) the leaves thus making them perk back up.

GTD
:thumbsup::thumbsup::thumbsup: well said.