How much light for my seedling?

My seedling is a week or so old, its 2nd set of siraded leaves are just coming in. I have heard that i can have it on a 24 hour light cycle, but some say 18 light a day is best. what difference does it make and what should i do? thanks

ok, so ima tell you rite now, it ALL works. it really depends on your setup. if you have really big lights that create alot of heat, you will want 18/6, which allows a cool off period. if you are running fluorescents, and DO NOT have a heat issue, then you will be fine with 24/0. hope i helped, grow on!

It is my belief that some dark for new seedlings allows a tiny bit of healthy stretch, which happens at night, as well as root formation.

I personally do 24/0 and keep my CFLs barely above the plants with a light breeze from a fan on them. I do not want height nor do I like stretching my growspace is small. As others have said...
18/6 is good if heat is an issue. I will for the first time have to switch to 18/6 this summer due to heat, I just have it turned off from about 3pm to 9pm the hottest part of the day here.

I dont know about root formation or stretching during the off cycle. I would assume with total blackness the plant would not stretch because it has no light to stretch towards. I would also think root formation continues regardless since its in solid dark under the dirt. But I do not know any of the last statement for sure, it just seems that way to me.

Good luck!

*edit* Outdoor I would assume the plant stretches at night because of the suns reflection on the moon. *edit*