My white widow autos did that all through veg on 18/6 - droopy by end of day. Budding out just fine. I think @cannabissequoia is right on point - simply tired - leaves may be using more water than transport can provide. During the dark transpiration and everything else slows down, and they perk up again.
Since cannabis doesn’t mind getting a little dry, I wouldnt worry about it and stay the course
