Hey weatherbox no problems switching from 18 to 20 hrs of light just a lot more growth. I dont know what it is but they love 20/4 I use to go 24 hrs but then I tried 18 and didnt get enough growth and went 20 bam explosive growth. Did you see the new pics in my thread thats 14 days of veg there very bushy everyday you can see growth. Just for shits and giggles try some liquid light you can pretty much what them grow right in front of ya.