Sounds to me like you're either using the wrong type of light, have it too far away, or a combination of both. These are generally the only reasons the stem would grow so long in such a short time. If you try growing under regular incandescent light bulbs (which are the wrong type), the stems will stretch right out like that. Also, if you have proper lights but you have it too far from the tops of the plants, stretching will occur then, too.
Try to keep the lights as close to the top of the plants as you can without burning them.
If you place your hand between the lights and the top of the plant and it gets hot, it's too close. If you can keep your hand there for a long period of time with no discomfort, then your plants won't feel discomfort, either. :)