I like to grow my own seedlings, but I'm never sure if the exposure should mimic the natural day length or if I should leave the lights on all the time.
Experiments show that more light gives better growth.
All visible wavelengths of light are used in photosynthesis. Hence, common fluorescent lights promote photosynthesis even though their light spectrum differs from the sun's. Plants respire in darkness, and some of the products of photosynthesis are transformed to other compounds. This explains why a dark period is not needed for seedling growth. I plant seeds in flats filled with commercial potting mix.