your traveling the wrong pathway.

The reason your plant is more happy is because it's getting more light. It's got nothing to do with how much dark it's getting in terms of speed of speed of growth. The plant is doing most of it's growing during lights on but will grow just a little more at night.

The difference in light is big compared with outside. Look into lumens and that line of research. There is plenty written on this that will explain what's going on and why your plant like the heavier light.