Either lights are too distanced away from the top of plant, therefore, It elongates to get the required amount or the color of the Grow-light is in the reddish side. 6400k is the ideal color. The interesting thing is that with red light, the plant will think that it’s in the natural shade so it kicks in its survival mode & elongates to reach the open space for better sun grasp. While the white light, the plant thinks that it’s in the open space, therefore will stay short and broad