my plants like low ppm 300 right now (4 weeks into veg.) so maybe in your case you are just finding the prefered levels (more isn't always better/ can cause nute burn). I think the plants like having a period of time when they arent recieving light, im no professor, but a break from photosynthesis might allow them to deal with it a little. give it 4 hours dark if you want faster growth, but mine are blowing up off 6 hours dark, and it's all about personal experience really. What works for me might not work for you, so at least you'll learn a lot when you make mistakes...... bla bla bla. happy growing!

-greens