Some plants, and quite a lot of seedlings, will grow in 24hr light. A light/dark cycle is necessary for most adult plants and is important to regulate flowering and fruiting. Tomatos are not induced to flower by an specific day length. I suggest 14-16 hours of light each day if the light is bright enough. Using fluorescents, which will not have the same intensity as direct sunlight, you might get better results leaving them on for longer.

i'm just fooling with growing under lights for the first time, and I don't plan to try tomatoes...this year. However, I've found that my seedlings all seem to be content with "before I take my shower in the morning" to "before I brush my teeth at night", whatever that happens to be.

Lisa

I've been using lights for seedlings for five or six years and I leave the lights on 24/7 because I'm lazy. My seedlings always seem to thrive...