So you have a big hps light on a plant and you also have a lot of foliage. What happens at night is the plants breathe on each other. If you go in and look when the light first comes on you might see moisture puddles. Those spots could show up later. I would try running a fan on low during the dark cycle. You don't need to blow it directly on the plants because you don't want to dry it out. Just get them wiggling a little. You might not need to blow it on them at all. That's about all I can tell you to try, unless you post a full sized image and turn the hps light off.