New grower, planted in stages 2 weeks apart, to avoid losing all plants/seeds due to inexperience. Glad I did that, because I lost about 8 plants in the first 3 weeks…
Anyway, I have successful plants that are a foot high and bushy (6 weeks old) now in the veg stage, under MHS lights on an 18/6 hr cycle. I also have 3-4 week old seedlings approx. 6" high with 2-4 sets of leaves, under a quad T5 array, that I also have on the same 18/6 cycle. So far so good… Its the very young seedlings that confuse me, the ones that just break through the peat pod with their first two embryonic leaves, and also the seedlings with just 1-2 sets of leaves and very weak stems…
My question is: should very young seedlings be under 24 hr light? or 18/6 cycle? They are all auto flowering strains but I’m not in a rush, I’d rather put them under a healthy cycle than try to push them to get bigger faster… that philosophy will probably change as I actually become more proficient.,
I have a 5 day old seedling right now under 24 hour lighting with a T5. I usually give young seedlings the first week with 24 hour lighting, then go down to 18/6 on the second week.
thank you, much appreciated
I have done it both ways and found in my case going right to a 16-8 schedule works great
I give them 24 hour light until they pop dirt and then to 16-8 schedule
I actually don’t use a 18-6 schedule anymore either
So yes you can leave them on as @raustin suggested 24 hours for the first week and then switch
It’s completely up to you
Thanks Countryboy, I’ve wondered myself about the 18/6 schedule… can’t think of many places that get 18 hours of daylight (excluding the North and South Pole, Alaska)…
All 24/0 ever got me was higher electric bills. 18/6, 16/8 both seem to work fine unless you want the extra warmth.
Seems to me that 18/6 even when i’m re-vegging a clone that the plant just does better. They look really beatdown on 24 hours , even on 20/4 mine wilt 
I’ve heard that the roots grow when the lights are on, and the tops in the dark, or vice-versa. But I think both are necessary. It’s a plant. I think that the 18/6 probably came about because of people wanting to ‘push the envelope’ and that was the longest they could go without doing damage.
For photosensitive plants I think a night cycle is crucial so going for a 18/6 or a 16/8 cycle will be as much light as you’ll want. Your plants won’t die under 24/0 but I don’t believe they’re be as vigorous or build the type of root system to produce the best quality possible quality flowers that the genetics are capable of. Now, if you’re growing _auto_flowering plants, it’s a different story - they do great under 24/0 from sprout to harvest. Lots of growers specializing in autos use 24/0 to maximize their harvest. The ruderalis genetics and the time clock they insert are gamechangers.
Hope this helps and happy growing!