Seedlings need light – lots of light. This photo was taken on March 2, and shows two tomato plants that were both raised under artificial light, 18 hours per day. The plant on the right was transplanted about 3 days before the photo was taken. You can see how much darker the plant on the left is, having received 18 hours of bright full-spectrum light, while the one on the right received much less light during the relatively short daylight in early March.
Before I had the lights and timer, my seedlings were tall and spindly. They were not strong enough to stand up straight and take full advantage of the sun. With a full 18 hours of light, the plants grow very sturdy stalks and make a much healthier plant.