They are stretching toward the light. They stretch the most for the first four or so weeks after you start them on the 12 hour cycle. After this, the stretching slows down or stops as the plant puts more energy into building up the buds. Some Sativas never stop stretching or growing.
Trust me it is good that your plants are stretching toward the lights, but just keep on eye on it and move your lights up as they get taller.
