I've grown with 24 hour veg times for 10 years now and I've never had any problems with it. I've ran side by side against 18/6 and couldn't notice any difference in the end. I have only vegged with T12's, T8's, T5's and CFL's though. If I was vegging with a 1000 watt MH, I'd be running 18/6 only...